FOURTEEN  WEEKS 


IN 


NATURAL   PHILOSOPHY, 


BY 

J.   DORMAN   STEELB,  PH.D., 

AUTHOR  OF  IHT8  »OtFBTMN-WB*K8  SKBIES  IN  PHYSIOLOGY,  CHEMISTRY, 
AOTBONOMY,  AND  OEOLOOT. 


•  The  works  of  God  are  fair  for  naught, 

Unless  our  eyes,  in  seeing, 
Bee  hidden  in  the  thing  the  tJwught 

That  animates  its  being."  — TILTON. 


A.    S.    BARNES    &    COMPANY, 

NEW  YORK,  CHICAGO  AND  NEW  ORLEANS. 


TIJE  FOURTEEN  WEEKS'  COURSES 

IN 

NATURAL    SCIENCE, 

BY 

J.  DORMAN    STEELE,  A.M.,  PH.D. 

Fourteeij  Weeks  iq  Natural  Philosophy,  j>  a 

Fourtee^  Weeks  iq  Cljei^istry,  (2  Editions)  /•  \j^  ' 

Fourteeij  Weeks  iq  Descriptive  J^stroijomy, 
Fourteen  Weeks  iq  Popular  Geology, 
Fourteen  Weeks  iq  Hunjan  Physiology, 

A  Key,  containing  Answers  to  the  Questions 
, .  ^n4  Problems  in  Stef  le's  14  Weeks'  Courses, 

"SERIES, 

"\&Teeks  in  the  Sciences, 


A  Brief  History  of  tip  United  States, 
A  Brief  ^istory  of  France, 

The  publishers  of  this  volume  will  send  either  of  the  above  by 
mail,  post-paid,  on  receipt  of  the  price. 

The  same  publishers  also  offer  the  following  standard  scientific 
works,  being  more  extended  or  difficult  treatises  than  those  of 
Prof.  Steele,  though  still  of  Academic  grade. 

Peck's  Gaijot's  Natural  Philosophy, 
Porter's  Principles  of  Cljeiqistry, 
Jarvis'  Physiology  aijd  Laws  of  tyealtfy 
Wood's  Botanist  aqd  Florist, 
Cljanjbers'  Elements  of  Zoology, 
tyclijtyre's  ^stroijomy  aqd  the  Globes, 
Page's  Elerqeijts  of  Geology, 

Address  A.  S.  BARNES  &  CO., 

Educational  Publishers, 

NEW  YORK  OR  CHICAGO. 

ENTERED  according  to  Act  of  Congress,  in  the  year  1869,  by 

A.    S.    BARNES    &    CO., 
In  the  Clerk's  Office  of  the  District  Court  of  the  United  States  for  the 

Southern  District  of  New  York. 
STEELE'S  NAT.  PHIL. 
* 

£DUCAT)ON  DEFT- 


TO 


Wife, 


IN  MY  SCIENTIFIC   STUDIES 

HAVE   BEEN 


IS 


d 

jlffectionatelg 

961666 


PEEFAOE. 


THIS  work  has  grown  up  in  the  class-room.  It 
contains  those  definitions,  .illustrations,  and  applica- 
tions which  seemed  at  the  time  to  interest  and  in- 
struct the  pupils.  Whenever  any  explanation  fixed 
the  attention  of  the  learner,  it  was.  laid  aside  for 
future  use.  Thus,  by  steady  accretions,  the  process 
has  gone  on  until  a  book  is  the  result. 

As  Physics  is  generally  the  first  branch  of  Natural 
Science  pursued  in  schools,  it  is  important  that  the 
beginner  should  not  be  disgusted  with  the  abstractions 
of  the  subject.  The  Author  has  therefore  endeavored 
to  use  such  simple  language  and  practical  illustrations 
as  will  interest  the  learner,  while  he  is  at  once  led 
out  into  real  life.  From  the  multitude  of  philosophi- 
cal principles,  those  only  have  been  selected  which 
are  essential  to  the  information  of  every  well-read 
person.  The  many  curious  questions  which  yet  rank 
only  as  "philosophical  gossip,"  are  rarely  mentioned. 
Within  the  brief  limits  of  a  small  text-book,  no  sub- 
ject can  be  exhaustively  treated.  This  is,  however, 
of  less  importance  now,  when  every  teacher  feels  that 
he  must  of  necessity  be  above  and  beyond  any  school- 
work  in  the  fulness  of  his  information. 


g  PREFACE. 

The  theories  advanced  are  those  generally  received 
among  scientific  men.  The  object  of  an  elementary 
work  is  not  to  advance  the  peculiar  ideas  of  any  one 
person,  but  simply  to  give  such  currently  accepted  facts 
as  are  believed  by  all.  This  plan  affords  no  scope  for 
original  thought.  The  Author  has  therefore  simply 
sought  to  gather  from  every  attainable  source  the  fresh- 
est and  most  valuable  information,  and  so  weave  it  to- 
gether as  to  please  as  well  as  instruct  his  pupils.  The 
time-honored  classifications  established  by  the  masters, 
and  recognized  in  all  scientific  works,  have  been  retained. 
It  has  not  seemed  wise  to  reject  familiar  terms  for  a 
mere  appearance  of  novelty.  Since  the  problems  are 
for  the  instruction  of  American  youth,  the  system  of v 
weights  in  general  use  in  this  country  is  employed. 

The  object  of  the  Author  will  be  fully  attained  if 
he  succeed  in  leading  some  young  mind  to  become  a 
lover  and  interpreter  of  Nature,  and  thus  come  at 
last  to  see  that  Nature  herself  is  but  a  "thought  of 
God." 


PECK'S  GANOT'S  NATURAL  PHILOSOPHY,  from  the  French  of 
A.  Ganot.  By  Prof.  W.  OK  Peck,  LL.D.,  Columbia  College. 
Adapted  to  follow  this  work,  or  take  its  place  in  classes  requiring 
a  fuller  course. 

"  Peck's  Ganot  "  is  noted  for  its  clear  method  and  its  magnifi- 
cent system  of  illustration  (from  the  original  French  plates), 
largely  doing  away  with  the  necessity  for  Apparatus. 

The  work  contains  500  12mo  pages,  and  is  of  Academic  grade. 
Price  $1.75,  post-paid,  A.  S.  BAKNES  &  Co.,  Publishers. 


SUGGESTIONS    TO    TEACHERS. 

SCHOLAKS  are  expected  to  obtain  information  from  this  book, 
without  the  aid  of  questions,  as  they  must  always  do  in  their 
general  reading.  When  the  subject  of  a  paragraph  is  announced, 
the  pupil  should  be  prepared  to  tell  all  he  knows  abput  it.  The 
diagrams  and  illustrations,  as  far  as  possible,  should  be  drawn  upon 
the  blackboard  and  explained.  Although  pupils  may,  at  first, 
manifest  an  unwillingness  to  do  this,  yet  in  a  little  time  it  will  be- 
come a  most  interesting  feature  of  the  recitation.  The  "  Practical 
Questions "  given  at  the  close  of  each  general  subject  have  been 
found  a  profitable  exercise  in  awakening  inquiry  and  stimulating 
thought.  They  may  be  used  at  the  pleasure  of  the  instructor.  The 
equations  contained  in  the  text  are  designed  to  be  employed  in  the 
solution  of  the  problems.  The  following  works  will  be  found  use- 
ful in  furnishing  additional  illustrations  and  in  elucidating  difficult 
subjects,  viz.:  Deschanel's  Natural  Philosophy,  Lockyer's  Guil- 
lemin's  Forces  of  Nature,  Stewart's  Elementary  Physics,  Herschel's 
Introduction  to  the  Study  of  Physical  Science,  Tomlinson's  Intro- 
duction to  the  Study  of  Natural  Philosophy,  Knight's  Cyclopedia — 
Section  on  Science  and  Natural  History,  Pepper's  Play-book  of 
Science,  Beale's  How  to  Work  with  the  Microscope,  Schellen's 
Spectrum  Analysis,  Lockyer's  The  Spectroscope,  Nugent's  Optics, 
Chevreul  on  Colors,  Thomson  &  Tait's  Natural  Philosophy,  Max- 
well's Electricity  and  Magnetism,  Faraday's  Forces  of  Matter, 
Youman's  Correlation  of  Physical  Forces,  Maury's  Physical  Geog- 
raphy of  the  Sea,  Atkinson's  Ganot's  Physics,  Silliman's  Physics, 
Tyndall's  Lectures  on  Light,  Heat  and  Sound,  Tyndall's  Forms  of 
Water,  Snell's  Olmsted's  Philosophy,  Loomis's  Meteorology,  Mil- 
ler's Chemical  Physics,  Cooke's  Religion  and  Cheiristry,  Benedicite, 
and  the  Illustrated  Library  of  Wonders.  They  may  be  procured 
of  the  publishers  of  this  book.  The  author  will  be  pleased  to  cor- 
respond with  teachers  with  regard  to  the  apparatus  necessary  foi 
the  performance  of  the  experiments  named  in  the  work,  or  with 
reference  to  any  of  the  "  Practical  Questions. " 


TABLE   OF   CONTENTS. 


I.    INTRODUCTION. 
•  P»»I 

MATTER .13 

GENERAL  PROPERTIES 15 

SPECIFIC  PROPERTIES 27 

II.  ATTRACTION    ...  33 

MOLECULAR  FORCES 35 

COHESION 36 

ADHESION 40 

GRAVITATION 48 

WEIGHT 

FALLING  BODIES 50 

CENTRE  OF  GRAVITY 54 

THE  PENDULUM 58 

III.    MOTION         .         .         .  68 

LAWS   OF  MOTION 70 

COMPOUND  MOTION 71 

COMPOSITION  AND  RESOLUTION  OF  FORCES       .  72 

CIRCULAR  MOTION 75 

REFLECTED  MOTION 79 

IV.    MECHANICAL  POWERS     .       .  83 

THE  ELEMENTS  OF  MACHINERY      ...  85 

LEVER 85 

WHEEL  AND  AXLE 90 

INCLINED  PLANE 93 

SCREW 95 

WEDGE 96 

PULLEY 97 


TABLE   OF   CONTENTS. 


11 


V.   PRESSURE   OF   LIQUIDS  AND   GASES  101 

HYDROSTATICS 103 

LIQUIDS  INFLUENCED  BY  PRESSURE    .        ,        .103 

LIQUIDS  INFLUENCED  BY  GRAVITY  ALONE     .        .  107 

SPECIFIC  GRAVITY 115 

HYDRAULICS      .         . 122 

WATER-WHEELS  .        .        .        .        .        .        .125 

WAVE-MOTION i?.8 

PNEUMATICS 132 

PROPERTIES  OF  THE  AIR 133 

PRESSURE  OF  THE  AIR,  BAROMETER,  PUMPS,  SI- 
PHON, &c 136 

VI.    ON  SOUND      ...  149 

ACOUSTICS 151 

SOUND  PRODUCED  BY  VIBRATIONS     .        .        .  151 

VELOCITY  OF  SOUND 155 

INTENSITY.     SPEAKING-TUBES,  TRUMPET,  &c.  .  157 

REFRACTION 159 

REFLECTION.    ECHOES,  &c 160 

NOISE  AND  Music 163 

THE  SIREN,  LENGTH  OF  SOUND-WAVE,  &c.       .  164 
INTERFERENCE  OF  SOUND      .        .        .        •        .168 

VIBRATION  OF  CORDS 169 

•NODES 171 

WIND  INSTRUMENTS 176 

THE  EAR 178 

SINGING  FLAMES 182 

VII.    ON  LIGHT      .        .       .  185 

OPTICS 187         * 

LAWS  AND  THEORY  OF  LIGHT        .       .       .       .188 

REFLECTION.    MIRRORS,  &c 190 

REFRACTION.    PRISMS,  &c.    .        .        .        «        .  197 
COMPOSITION.     THE    SPECTRUM,  DIFFRACTION, 

POLARIZATION,  THE  RAINBOW,  &c.     .        .  204 


12  TABLE  OF  CONTENTS. 

OPTICAL  INSTRUMENTS.  TELESCOPE,  MICROSCOPE, 
OPERA-GLASS,  MAGIC  LANTERN,  STEREOSCOPE, 
CAMERA,  &c 215 

THE  EYE     .       .       .       .       .       .       .       .220 

VIII.    ON  HEAT       .         .       .  225 

NATURE  AND  THEORY  OF  HEAT      .        .        .228 
CHANGE  OF   STATE  BY  HEAT         ...      232 

EXPANSION 233 

LIQUEFACTION 236 

VAPORIZATION 237 

EVAPORATION 241 

COMMUNICATION   OF   HEAT         .        .         .         .243 

CONDUCTION 243 

CONVECTION 244 

RADIATION 245 

THE   STEAM-ENGINE 246 

METEOROLOGY 248 

IX.    ON  ELECTRICITY       .        .       259 

MAGNETIC  ELECTRICITY 261 

FRICTIONAL  ELECTRICITY          ....       269 

GALVANIC  ELECTRICITY 287 

ELECTRO-MAGNETISM 302 

ANIMAL  ELECTRICITY 314 

CONCLUSION 314 

NOTES  ON  APPARATUS  AND  EXPERIMENTS   .  318 

QUESTIONS         .      •  . 523 

INDEX  •        •  .      339 


NATURAL    PHILOSOPHY. 


INTRODUCTION. 

MATTER  is  anything  we  can  perceive  with  our 
senses.  A  body  is  a  distinct  portion  of  matter.  Ex. : 
A  chair,  1  Ib.  of  iron,  a  piece  of  silver.  A  substance 
is  any  one  of  the  different  kinds  of  matter.  Ex. : 
Gold,  wood,  stone. 

PROPERTIES  OF  MATTER. — Each  substance  possesses 
two  kinds  of  properties — general  and  specific  ;  the 
former  belongs  to  all  substances,  the  latter  only  to 
particular  ones.  Ex. :  Gold  has  weight.  This  prop- 
erty is  not  peculiar  to  gold,  for  all  substances  have 
weight ;  hence  it  is  a  general  property.  But  gold  is 
yellow.  This  is  so  distinctive  that  we  speak  of  a 
"  golden  color ;"  hence  it  is  a  specific  property.  A 
piece  of  glass  has  form.  All  bodies  have  form ; 
hence  it  is  a  general  property.  But  glass  is  so  brittle 
that  we  say  "  brittle  as  glass ;"  hence  brittleness  is 
a  specific  property. 


I4  NATURAL  PHILOSOPHY. 

CHANGES  OF  MATTEE. — Each  substance  can  un- 
dergo two  kinds  of  .change — physical  and  chemical. 
Tije"  fbrmeiLdQeV  iiqt  destroy  the  specific  properties 
of  ,tliQ  cu-hstance-;  ;the  latter  does.  Ex.:  An  eagle 
is  beaten  into  'gold-leaf ;  but  this  does  not  alter  the 
color  or  other  specific  properties  of  the  metal,  hence 
it  is  a  physical  change.  Melt  the  eagle  in  a  cruci- 
ble, and  still  the  same  is  true.  But  put  it  into  an 
acid,  and  soon  it  is  dissolved — the  specific  proper- 
ties are  entirely  destroyed ;  hence  it  is  a  chemical 
change.  Draw  a  nail  into  wire,  and  the  specific 
properties  of  the  iron  remain  untouched.  But  leave 
it  in  a  basin  of  water,  and  the  distinguishing  proper- 
ties of  iron  disappear — it  becomes  brittle,  red,  soft, 
and  scaly ;  hence  this  is  a  chemical  change. 

These  distinctions  give  rise  to  two  branches  oi 
Natural  Science,  PHILOSOPHY  and  CHEMISTKY.  The 
former  treats  of  the  physical,  the  latter  of  the 
chemical  changes  of  matter.  As  all  bodies  possess 
both  kinds  of  properties,  and  are  susceptible  of 
both  kinds  of  change,  these  branches  are  inti- 
mately connected. 

fraclicat  Questions .— My  knife-blade  is  magnetized,  so  that  it  will  pick 
up  a  needle :  is  that  a  physical  or  chemical  change  ?  Is  it  treated  in  Phi- 
losophy or  Chemistry? 

Is  the  burning  of  coal  a  physical  or  chemical  change  ?  The  production 
of  steam  ?  The  formation  of  dew  ?  The  falling  of  a  stone  ?  The  growth 
of  a  tree  ?  The  flying  of  a  kite  ?  The  chopping  of  wood  ?  The  explosion 
of  powder  ?  The  boiling  of  water  ?  The  melting  of  iron  ?  The  drying  of 
clothes  ?  The  freezing  of  water  ? 


GENERAL  PR  OPER  TIES.  i  5 

THE  GENERAL  PROPERTIES  OF  MATTER. 

'We  cannot  imagine  a  body  which  does  not  pos- 
sess all  the  general  properties  of  matter.  The  most 
important  are  Magnitude,  Impenetrability,  Divisi- 
bility, Porosity,  Inertia,  and  Indestructibility. 

MAGNITUDE  is  the  property  of  occupying  space. 
Size  is  the  amount  of  space  a  body  fills.  Every 
body  has  three  dimensions — length,  breadth,  and 
thickness.  In  order  to  measure  these,  some 
standard  is  required.  Anciently,  certain  portions 
of  the  human  body  were  used  for  this  purpose. 
Ex. :  The  foot ;  the  cubit,  or  length  of  the  forearm 
from  the  elbow  to  the  end  of  the  middle  finger ;  the 
finger's  length  or  breadth  ;  the  hand's  breadth  ;  the 
span,  etc. 

The  English  system. — The  first  intimation  that  is 
given  of  an  attempt  to  have  a  standard  in  England, 
is  that  of  1120.  King  Henry  ordered  that  the  ell, 
the  ancient  yard,  should  be  the  exact  length  of  his 
arm.  Afterward  we  learn  that  a  standard  yard- 
stick was  kept  at  the  Exchequer  in  London ;  but  it 
was  so  inaccurate,  that  a  commissioner,  who  exam- 
ined it  in  1742,  wrote :  "  A  kitchen  poker  filed  at 
both  ends  would  make  as  good  a  standard.  It  has 
been  broken,  and  then  repaired  so  clumsily  that  the 
joint  is  nearly  as  loose  as  a  pair  of  tongs."  In 
1760,  Mr.  Bird  prepared  an  accurate  copy  of  this 
for  the  use  of  the  Government.  It  was  not  legally 
adopted  until  1824,  when  it  was  ordered,  that  if  de~ 


1  6  NA  TUBAL  PHIL  OSOPHY. 

stroyed  it  should  be  restored  by  a  comparison  with 
the  length  of  a  pendulum  vibrating  seconds  at  the 
latitude  of  London.  (See  page  59,  4th  Law.) 

At  the  great  fire  in  London,  1834,  the  Parliament 
House  was  burned,  and  with  it  Bird's  yard-stick. 
Repeated  attempts  were  then  made  to  find  the  length 
of  the  lost  standard  by  means  of  the  pendulum. 
This  was  found  utterly  impracticable.  At  last  the 
British  Government  adopted  a  standard  prepared 
from  the  most  reliable  copies  of  Bird's  yard-stick. 
A  copy  of  this  was  taken  by  Troughton,  a  celebrated 
instrument-maker  of  London,  for  the  use  of  our 
Coast  Savvey.* 

The  2''fr.rtch  Standard.  —  The  French  adopted  as 
the  length  of  the  legal  foot,  that  of  the  royal  foot 
of  King  Louis  XIV.,  as  perishable  a  standard 
as  King  Henry's  arm.  In  1790,  however,  they 
took  Tir,¥oV,oTro  °i'  ^ne  length  of  the  quarter  of  a  me- 
ridian of  the  earth's  circumference  as  the  basis  of 
all  measurements.  This  is  equal  to  39  +  inches, 
and  is  called  a  metre. 

A  Natural  Standard.  —  Two  attempts  have  thus 
been  made  to  fix  upon  something  in  Nature  as  an 
invariable  unit  of  measure.  The  French  had,  how- 
ever, scarcely  completed  their  system,  when  they 
found  that  a  mistake  had  been  made  in  measuring 


*  This  yard  is  about  noT  of  an  inch  longer  than  the  British 
standard.  According  to  Act  of  Congress,  sets  of  weights  and 
measures  have  been  distributed  to  the  governors  of  the  several 
states.  The  yards  so  furnished  are  equal  to  that  of  the  Troughton 
scale.  We  have  no  national  standard  established  by  law. 


GENERAL  PROPERTIES.  !  7 

the  meridian.  The  English  philosophers  discovered 
a  similar  error  in  the  calculation  of  the  length  of  the 
pendulum.  Both  the  French  and  English  systems 
are  therefore  founded  upon  arbitrary  standards. 

IMPENETRABILITY  is  the  property  of  so  occupying 
space  as  to  exclude  all  other  bodies.  No  two 
bodies  can  occupy  the  same  space  at  the  same 
time.  A  book  lies  upon  the  table  before  me ;  no 
power  in  the  world  is  able  to  place  another  in  the 
same  space  with  that.  I  attempt  to  fill  a  bottle 
through  a  closely-fitting  funnel ;  but  before  the 
liquid  can  run. in,  the  air  must  gurgle  out  or  the 
water  will  trickle  down  the  sides  of  the  bottle. 

Apparent  Exceptions. — In  common  language,  we 
speak  of  one  substance  penetrating  another.  Thus, 
a  needle  penetrates  cloth,  a  nail  penetrates  wood, 
etc. ;  but  a  moment's  examination  shows  that  they 
merely  push  aside  the  fibres  of  the  cloth  or  wood, 
and  so  press  them  closer  together.  Take  a  tum- 
bler brim-full  of  water,  and  then  cautiously  drop 
in  shingle-nails.  A  quarter  of  a  pound  can  be 
easily  added  without  causing  the  water  to  over- 
flow. We  shall  find  the  explanation  of  this,  in 
the  fact  that  the  surface  of  the  water  becomes  con- 
vex. 

DIVISIBILITY  is  that  property  of  a  body  which  al- 
lows it  to  be  separated  into  parts.  We  have  never 
seen  a  particle  so  small  that  we  could  not  make 
it  smaller. 

Illustrations.  —  The  thread  with  which  certain 
species  of  spiders  weave  their  web  is  composed  of 


jg  NATURAL  PHILOSOPHY. 

four  smaller  threads ;  each  one  of  these  consists  of 
one  thousand  yet  smaller,  each  of  which  comes 
from  a  separate  tube  in  the  spider's  spinning- 
machine.  A  German  naturalist,  after  examining 
a  web  very  carefully,  decided  that  it  would  take 
4,000,000,000  fibres  to  form  a  thread  as  large 
as  a  hair  of  his  beard ;  and  as  each  fibre  is  com- 
posed of  4,000  smaller  ones,  it  follows  that  each  of 
the  least  fibres  is  only  T-g-, -QOT, tnA, mnr, mr "o  Part  the  size 
of  a  human  hair.  It  is  said  that  a  half-pound  of  the 
full-sized  thread  would  girt  the  globe.  It  would  re- 
quire 50,000,000  pounds  of  wire  to  erect  a  telegraph 
around  the  earth  at  the  equator.  A  grain  of  strych- 
nine will  impart  a  flavor  to  1,750,000  grains  of 
water  ;  hence,  each  grain  of  the  liquid  will  contain 
only  T,TTCT,WO-  °f  a  grain  of  strychnine,  and  yet  that 
amount  can  be  distinctly  tasted.  A  grain  of  Ma- 
genta will  color  50,000,000  grains  of  water.  A  piece 
of  silver  containing  only  one-billionth  of  a  cubic 
inch — i.  e.,  being  .001  of  au  inch  square — when  dis- 
solved in  nitric  acid  will  render  milky  a  solution 
of  a  hundred  cubic  inches  of  common  salt.  Each 
cubic  inch  must  then  contain  TTrff.TfTn&nnr.Tnnr  of  a 
cubic  inch  of  silver.  The  eye  can  see  the  color 
distinctly  in,  perhaps,  a  hundredth  part  of  a  cubic 
inch,  in  which  there  would  be  present  only  one  ten- 
trillionth  part  of  a  cubic  inch  of  silver.* 

*  Some  idea  of  the  vastness  expressed  by  the  word  trillion 
may  be  derived  from  the  following  curious  calculation.  If  Adam, 
at  the  instant  of  his  creation,  had  commenced  to  count  at  the 


GENERAL  PROPERTIES.  lg 

Press  a  puff-ball,  and  each  speck  of  the  cloud 
which  flies  off,  under  the  microscope  proves  to  be 
a  beautiful  round  orange-ball, — the  seed  of  the 
plant.  Much  of  the  fine  dust  that  is  revealed  to  us 
hi  the  atmosphere,  by  a  beam  of  light  shining 
through  a  crevice,  consists  of  the  seeds  of  minute 
plants,  which  falling  on  a  damp  surface  grow  into 
mildew  or  mould.  Under  a  microscope,  this  be- 
comes a  fairy  forest  of  trees  of  a  new  and  strange 
growth. 

In  all  these  instances  we  have  mentioned,  the 
divisibility  is  proved  by  our  senses  of  taste  and 
sight.  When  our  eyes  fail,  the  microscope  is  called 
in  to  continue  the  investigation.  "While  thus,  prac- 
tically, there  is  no  limit  to  the  divisibility  of  matter, 
philosophers  hold  that  there  is,  in  theory. 

THE  ATOMIC  THEORY  supposes  that  matter  is 
composed  of  inconceivably  minute  portions  called 
atoms,  each  having  a  definite  shape,  weight,  color, 
etc.,  which  cannot  be  changed  by  any  chemical  or 
physical  force.  As  has  been  happily  said,  "  What 
God  made  one  in  the  beginning,  man  cannot  put 
asunder."  No  one  has  ever  seen  one  of  these  ulti- 
mate portions  of  matter,  and  we  have  no  absolute 
proof  that  any  exist ;  but  the  theory  is  so  conve- 


rate  of  one  every  second  of  time,  continuing  through  all  the  cen- 
turies, he  would  not  yet  have  nearly  completed  the  first  quarter 
of  a  trillion  ;  and  even  if  Eve  had  come  to  his  relief,  and  together 
they  had  counted  day  and  night,  they  would  not  see  the  end  of 
their  task  and  enjoy  their  first  leisure  for  10,000  years  to  come. 


20  NATURAL  PHILOSOPHY. 

nient,  especially  in  chemistry,  that  it  is  at  present 
generally       7eived. 

ANIMALCULE. — The  tiny  nations  of  animalculse  fur- 
nish mos  striking  illustrations  of  the  divisibility  of 
matter  rud.  the  minuteness  of  atoms.  This  is  a 
world  of  which  our  unaided  senses  furnish  us  no 
'proof.  The  microscope  alone  reveals  its  wonders. 
In  the  drop  of  water  that  clings  to  the  point  of  a 
cambric  needle,  the  swarming  millions  of  this  min- 
iature world  live,  grow,  and  die.  They  swim  in 
this  their  ocean,  full  of  life,  frisking,  preying  upon 
each  other,  waging  war,  and  re-enacting  the 
scenes  of  the  great  world  we  see  about  us.  My- 
riads of  them  inhabit  the  pools  of  water  standing 
along  the  roadside  in  summer.  They  go  up  in 
vapor  and  fly  off  in  dust,  and  reappear  wherever 
moisture  and  heat  favor  the  development  of  life. 
Yet,  minute  as  they  are,  they  have  been  fossilized 
(turned  to  stone),  and  now  form  masses  of  chalk. 
Tripoli,  or  polishing-slate,  is  composed  of  these  re- 
mains, each  skeleton  weighing  the  TTT,TTOIT,TO-O  of  a 
grain.  If  we  examine  whiting  under  a  powerful 
microscope,  we  shall  find  that  it  is  composed  of  tiny 
sMls.  Now  let  our  imagination  conceive  the  minute 
animals  which  formerly  occupied  them.  Many  of  them 
had  simple  sack-like  bodies,  but  still  they  had  one  or 
more  stomachs,  and  possessed  the  power  of  digesting 
and  assimilating  food.  This  food,  coursing  in  infinitely 
minute  channels,  must  have  been  composed  of  solid  as  well 


GENERAL  PROPERTIES.  21 

as  liquid  matter ;  and  finally,  at  the  lowest  extreme 
of  this  descending  series,  we  come  to  the  atoms  of 
which  this  matter  itself  was  composed. 

POROSITY  is  the  property  of  having  pores.  By 
this  is  meant  not  only  such  pores  as  are  familiar  to  us 
all,  and  to  which  we  refer  when  in  common  language 
we  speak  of  a  porous  body,  as  bread,  wood,  un- 
glazed  pottery,  a  sponge,  etc.,  but  a  finer  kind, 
which  are  as  invisible  to  the  eye  as  the  atoms  them- 
selves. These  pores  are  caused  by  the  fact  that  the 
molecules*  of  which  a  body  is  composed,  are  not  in 
actual  contact,  but  are  separated  by  extremely 
minute  spaces. 

Size  of  the  spaces  compared  with  the  size  of  tJie  atoms. 
—These  spaces  are  so  small  that  they  cannot  be  dis- 
cerned with  the  most  powerful  microscope,  yet  it  is 
thought  that  they  are  very  large  as  compared  with 
the  size  of  the  atoms  themselves.  If  we  imagine  a 
being  small  enough  to  live  on  one  of  the  atoms  near 
the  centre  of  a  stone,  as  we  live  on  the  earth,  then 
we  are  to  suppose  that  he  would  see  the  nearest 
atoms  at  great  distances  from  him,  as  we  see  the 
moon  and  stars,  and  might  perchance  have  need  of 
a  fairy  telescope  to  examine  them,  as  we  investigate 
the  heavenly  bodies. 

Illustrations. — 1.   Having  a  bowl  full  of  water,  it 

*  The  word  molecule  means  a  little  mass.  A  group  of  atoms 
forms  a  molecule,  and  a  collection  of  molecules  constitutes  a  body. 
Thus  a  molecule  of  water  is  composed  of  two  atoms  of  hydrogen 
and  one  of  oxygen.  (See  New  Chemistry,  Rev.  Ed.,  p.  56.) 


22  NA  TURAL  PHILOSOPHY. 

is  easy  to  add  a  large  quantity  of  fine  salt  without 
apparently  increasing  the  bulk  of  water  in  the 
least.  We  must  only  be  careful  to  drop  in  the 
salt  slowly,  giving  it  time  to  dissolve  and  the  bub- 
bles of  air  to  pass  off.  When  the  liquid  has  taken 
up  all  the  salt,  we  can  add  finely  powdered  sugar, 
and  afterward  other  soluble  solids  in  the  same  man- 
ner. In  this  case  we  suppose  that  the  particles  of 
sugar  are  smaller  than  those  of  salt,  and  those  in 
turn  smaller  than  those  of  water.  The  particles  of 
salt  fill  the  spaces  between  the  particles  of  water, 
and  those  of  sugar  occupy  the  still  smaller  spaces 
left  between  the  particles  of  salt.  We  may  better 
understand  this  if  we  suppose  a  bowl  filled  with 
oranges.  It  will  still  hold  a  quantity  of  peas,  then 
of  gravel,  then  of  fine  sand,  and  lastly  some  water. 

2.  At  Florence,  Italy,  in  the  17th  century,  a  hol- 
low sphere  of  gold  was  filled  with  water  and  tightly 
closed.     Pressure  was  then  applied  to  the  outside, 
and  the  ball  partly  flattened.     This  change  of  form 
diminished  the  size,  and  so  the  water  was  forced 
through  the  metal  and  formed  on  the  surface  like 
drops  of  dew.     This  experiment  proved  that  gold 
has  pores,  and  that  they  are  larger  than  the  mole- 
cules of  water. 

3.  In  testing  large  cannon,  water  is  forced  into 
the    gun   by    hydrostatic    pressure    until    it    oozes 
through  the  thick  metal  and  covers  the  outside  of 
the   gun   like   froth,  then   gathers   into   drops  and 
runs  down  to  the  ground  in  streams. 


GENERAL  PROPERTIES.  2^ 

4.  Over  the  Menai  Straits  there  is  a  magnificent 
tubular  bridge  100  feet  above  the  water.     The  tubes 
were  floated  to  the  spot  in  vessels,  and  then  raised 
to  their  position  by  means  of  immense   Hydraulic 
Presses.     The  cylinders  of  these  presses  (see  R,  Fig. 
70)  were  made  of  iron  a  foot  thick.     Yet  when  in 
full  operation  it  is  said  the  water  would  form  in 
drops  on  the  outside,  and  the  workmen,  rubbing  it 
off  with  their  fingers,  would  speak  of  the  machine  as 
"  sweating."     They  were  at  last  compelled  to  partly 
stop  these  pores  by  mixing  oat-meal  with  the  water 
used  in  the  presses. 

5.  Stone  pillars   and  arches  are  frequently  com- 
pressed by  the  great  weight  which  rests  upon  them. 
The  columns  which  support  the  dome  of  the  Pan- 
theon at  Paris  are  said  to  have  been  considerably 
shortened  in  this  manner. 

6.  Ashes  will  "  keep  fire  "  because  they  are  por- 
ous,  and  permit   enough   air   to  pass  in  to  main- 
tain  a  slow  combustion  and  so  preserve  the  coals 
alive. 

7.  The  process  of  filtering,  so  much  employed  by 
druggists  and  chemists,  depends  upon  this  property ; 
the  liquid  slowly  passes  through  the  pores  of  the 
filter,  leaving  the  solid  portions  behind.     Water,  in 
Nature,  is  thus  purified  by  percolating  through  beds 
of  sand  and  gravel.     Cisterns  for  filtering  water  have 
a  partition  in  the  middle ;  one  side  contains  charcoal 


24 


NATURAL  PHILOSOPHY. 


and  sand,  the  other  the  rain-water ;  as  the  water 
filters  through  these  substances  it  is  cleansed  of  its 
impurities.  Small  niters  are  frequently  made  on  the 

Fig.  1. 


same  principle.  They  consist  of  a  cask  nearly  filled 
with  gravel  and  charcoal;  the  water  is  poured  in  a 
little  reservoir  at  the  top  and  drawn  off  at  the  bot- 
tom by  a  faucet. 

8.  Gases  are  known  to  be  porous  from  the  fact 
that  when  a  jar  is  filled  with  one  kind  of  gas,  it  will 
contain  as  much  of  another  kind  as  if  the  jar  were 
empty.  The  molecules  of  the  second  must  spread 
themselves  between  the  molecules  of  the  first.  This 
illustrates  the  principle  that  "one  gas  is  a  vacuum 
for  another  gas." 


GENERAL  PROPERTIES.  2^ 

INERTIA  is  the  property  of  passiveness.  Matter 
has  no  power  of  putting  itself  in  motion  when  at  rest, 
nor  of  coming  to  rest  when  in  motion.  A  body  will 
never  change  its  place  unless  moved,  and  if  once 
started  will  move  forever  unless  stopped.  If  we 
leave  our  room,  and  on  our  return  find  a  book  miss- 
ing, we  know  some  one  has  taken  it, — the  book  could 
not  have  gone  off  at  its  own  suggestion.  We  gen- 
erally think  a  body  is  more  inclined  to  rest  than  to 
motion ;  and  so,  while  we  see  how  a  stone  could* not 
throw  itself,  we  find  it  difficult  to  understand  how, 
once  thrown,  it  does  not  stop  itself.  We  shall  see 
hereafter  that  several  forces  destroy  its  motion  and 
bring  it  to  rest. 

Illustrations. — 1.  When  we  try  to  start  a  heavy 
wagon  it  requires  a  great  effort,  because  we  have  to 
overcome  its  inertia,  which  tends  to  keep  it  at  rest. 
When  the  wagon  is  in  motion  it  requires  as  great  an 
exertion  to  stop  it,  since  then  we  have  again  to  over- 
come its  inertia,  which  tends  to  keep  it  moving. 

2.  Inertia  causes  the  danger  in  jumping  from  a 
car  when  in  rapid  motion.  The  body  has  the  speed 
of  the  train,  while  the  motion  of  the  feet  is  stopped 
by  the  contact  with  the  ground.  One  should  jump 
as  nearly  as  he  can  in  the  direction  in  which  the 
train  is  moving,  and  with  his  muscles  strained,  s« 
as  to  break  into  a  run  the  instant  his  feet  touch 
the  ground.  Then  with  all  his  strength  he  can 
gradually  overcome  the  inertia  of  his  body,  and 
after  a  few  rods  can  turn  as  he  pleases. 


26  NATURAL  PHILOSOPHY. 

Practical  Questions.— \.  If  one  is  riding  rapidly,  in  which  direction  will 
he  be  thrown  when  the  horse  is  suddenly  stopped  ?  2.  When  standing  in  a 
boat,  why,  as  it  starts,  are  we  thrown  backward?  3.  When  carrying  a  cup  of 
tea,  if  we  move  or  stop  quickly,  why  is  the  liquid  liable  to  spill  ?  4.  Why, 
when  closely  pursued,  can  we  escape  by  dodging?  5.  Why  is  a  carriage  or 
sleigh,  when  sharply  turning  a  corner,  liable  to  tip  over?  6.  Why,  if  you 
place  a  card  on  your  finger  and  on  top  of  it  a  cent,  can  you  snap  the  card  from 
under  the  cent  without  knocking  the  latter  off  your  finger  ? 

INDESTKUCTIBILITY  is  the  property  which  renders 
matter  incapable  of  being  destroyed.  No  particle 
of  matter  can  be  annihilated,  except  by  God,  its 
creator.  We  may  change  its  form,  but  we  cannot 
deprive  it  of  existence.  Ex.  :  We  cut  down  a  tree, 
saw  it  into  boards,  and  build  a  house.  The  house 
burns,  and  only  little  heaps  of  ashes  remain  behind. 
Yet  in  these  ashes,  and  in  the  smoke  of  the  burning 
building,  exist  the  identical  atoms  which  have  passed 
through  these  various  forms  unchanged  in  shape, 
color,  or  weight.* 

Compressibility  is  often  given  as  a  general  prop- 
erty of  matter.  It  is,  however,  a  mere  result  and 
proof  of  porosity.  It  is  a  distinguishing  feature  of 
gases.  They  are  readily  compressed ;  solids  require 
more  force,  while,  for  a  long  time,  this  property  was 
denied  to  liquids,  and  they  are  even  now  practically 
incompressible.  Weight  is  also  a  property  of  all 

*  Sir  Walter  Raleigh,  while  smoking  in  the  presence  of  Queen 
Elizabeth,  offered  to  bet  her  majesty  that  he  could  tell  the  weight 
of  the  smoke  that  curled  upward  from  his  pipe.  The  bet  was  ac- 
cepted. Raleigh  quietly  finished,  and  then  weighing  the  ashes, 
subtracted  this  amount  from  the  weight  of  the  tobacco  he  had 
placed  in  the  pipe  ;  he  thus  found  the  exact  weight  of  the  smoke. 
The  queen  is  said  to  have  paid  the  wager,  having  in  this  way 
learned  something  of  the  indestructibility  of  matter. 


GENERAL  PR  OPER  TIES.  2  y 

bodies  with  which  we  are  acquainted.  It  is  not, 
however,  essential  to  our  idea  of  matter,  since  we 
can  conceive  of  a  substance  without  weight.  "Weight 
is  only  the  result  of  attraction.  Indeed,  if  there 
were  but  one  body  in  the  universe  it  would  have  no 
weight,  since  it  could  not  be  attracted  in  any  direction. 

THE  SPECIFIC  PROPERTIES  OF  MATTER. 

THESE  are  properties  which  are  found  only  in  par- 
ticular kinds  of  matter.  The  most  important  are 
Ductility,  Malleability,  Tenacity,  Elasticity,  Hard- 
ness, and  Brittleness.  They  are  doubtless  caused  by 
modifications  in  the  attraction  of  Cohesion,  of  which 
we  shall  soon  speak. 

DUCTILITY. — A  ductile  body  is  one  which  can  be 
drawn  into  wire.  In  the  cut  is  represented  a  machine 

Fig.  2. 


for  making  iron  wire.  B  is  a  steel  drawing-plate 
pierced  with  a  series  of  gradually  diminishing  holes. 
A  rod  of  iron,  A,  is  hammered  at  the  end  so  as  to 
pass  through  the  largest  of  these.  It  is  then  grasped 
by  a  pair  of  pincers,  C,  and,  by  turning  the  crank,  D, 
is  drawn  through  the  plate,  diminished  in  size  and 
proportionately  increased  in  length.  The  speed 


28  NATURAL  PHILOSOPHY. 

varies  from  one  to  six  feet  per  second,  according  to 
size  and  quality.  The  rod  is  then  passed  through 
a  smaller  hole ;  and  the  process  is  continued  until  the 
required  fineness  is  reached.  The  holes  in  the  plate 
are  kept  well  lubricated  with  grease  or  wax.  The 
wire  is  strongest  when  drawn  cold.  After  a  few 
drawings  the  iron  loses  in  part  its  ductility,  and  is 
then  annealed  by  being  heated  in  an  oven  and  after- 
ward cooled  slowly.  The  tenacity  of  iron  is  in- 
creased by  the  process  of  drawing.  A  bar  one  inch 
square,  which  would  sustain  30  tons,  on  being  con- 
Verted  into  a  coarse  wire  rope  will  sustain  40  tons, 
and  into  fine  wire,  even  90  tons. 

Gold,  silver  and  platinum  are  the  most  ductile 
metals.  A  silver  rod  an  inch  thick,  covered  with 
gold-leaf,  may  be  drawn  to  the  fineness  of  a  hair  and 
yet  retain  a  perfect  coating  of  gold — 3  oz.  of  the 
latter  metal  making  100  miles  of  the  gilt-thread  used 
in  embroidery.  Platinum  wire  has  been  drawn  so 
fine  that,  though  it  is  the  densest  of  the  metals  used 
for  this  purpose,  being  nearly  three  times  as  heavy  as 
iron,  a  mile's  length  weighed  only  a  single  grain. 
(See  Eevised  Chem.,  page  170,  for  description  of  the 
process.)  Brass  wire  is  made  so  small,  that  when 
woven  into  gauze  there  are  67,000  meshes  in  a  square 
inch. 

MALLEABILITY. — A  malleable  body  is  one  which 
can  be  hammered  or  rolled  into  sheets.  Gold  is 
one  of  the  most  malleable  of  metals.  Gold-leaf  is 
prepared  in  the  following  manner.  An  ingot  of 


SPECIFIC  PROPERTIES.  2O 

gold  is  passed  many  times  between  steel  rollers, 
which  are  so  adjusted  as  to  be  brought  constantly 
nearer  together.  An  ounce  of  gold  is  thus  reduced  to 
a  ribbon  one  inch  wide  and  15  feet  long.  This  is 
cut  into  pieces  an  inch  in  length.  150  of  these  are 
piled  up  alternately  with  leaves  of  strong  paper  four 
inches  square.  A  workman  with  a  heavy  hammer 
beats  this  pile  until  the  gold  is  spread  to  the  size  of 
the  leaves.  Each  piece  is  next  quartered  and  the 
600  squares  are  placed  between  leaves  of  gold- 
beater's skin  and  re-pounded.  They  are  then  taken 
out,  spread  by  the  breath,  re-cut,  and  the  2,400 
squares  re-pounded  as  before.  The  beating  may  be 
continued  until  360,000  leaves  make  only  an  inch  in 
thickness.  They  are  finally  trimmed  and  placed 
between  the  pages  of  little  books,  each  of  which 
contains  25  gold  leaves. 

Copper  is  so  malleable,  that  it  is  said  that  a  work- 
man, with  his  hammer,  can  beat  out  a  kettle  from  a 
solid  block  of  the  metal. 

TENACITY. — A  tenacious  body  is  one  which  cannpt 
be  easily  pulled  apart.  Iron  is  the  most  tenacious 
of  the  metals.  A  wire  .078  of  an  inch  in  diameter, 
will  sustain  a  weight  of  nearly  450  Ibs.,  while  one  of 
lead  would  be  broken  by  a  weight  of  28  Ibs. 

ELASTICITY  is  of  three  kinds :  Elasticity  of  Com- 
pression, Elasticity  of  Expansion,  and  Elasticity  of 
Torsion,  ^according  as  a  body  tends  to  resume  its 
original  form  when  compressed,  extended,  or  twisted. 

Elasticity  of  Compression. — 1.  Many  solids  possess 


NATURAL  PHILOSOPHY. 


this  property  in  a  high  degree.     A  sword  was  ex- 
hibited at  the  World's  Fair  in  London,  which  could 

be  bent  into  a  circle,  and 
on  being  released  would 
fly  back  and  become 
straight  again.  The  elas- 
ticity of  ivory  may  be 
shown  by  the  following 
experiment.  Spread  a 
thin  coat  of  oil  on  a 
smooth  marble  slab.  If 
an  ivory  ball  be  dropped 
upon  it,  the  size  of  the 
impression  made  will 
vary  with  the  distance  at 
which  the  ball  is  held 
above  the  table.  This 
shows  that  the  ivory  is 
flattened,  somewhat  as  is  a  soap-bubble  when  it 
strikes  a  smooth  surface  and  rebounds.  Putty  and 
clay  are  slightly  elastic.  2.  Liquids  are  compressed 
with  great  difficulty  ;*  but  when  the  force  is  removed 
they  regain  their  exact  volume.  They  are  therefore 
oerfectly  elastic.  3.  Gases  are  easily  compressed, 


*  Thus,  for  a  pressure  of  one  atmosphere,  15  Ibs.  per  square  inch, 
the  diminution  of  volume  of  the  following  liquids  is  only,  as  com' 
pared  with  the  original  volume — 

1.  Water,        T,ooi,VoT        4-  Alcohol>          T,W*,W 

2.  Mercury,     T,-(nn),iro  o         5-  Chloroform, 

3.  Ether,          ^^^^        6>  gea-water, 


SPECIFIC  PROPERTIES. 


but  are  also  perfectly  elastic.  A  pressure  of  15  Ibs. 
to  the  square  inch  reduces  the  bulk  of  water  only 
TovWo  part,  whereas  it  diminishes  the  volume  of  a 
gas  one-half.  A  gas  may  be  kept  compressed  for 
years,  but  will  instantly  return  to  its  original  form 
on  being  released, 

Elasticity  of  Expansion. — This  property  is  possessed 
largely  by  solids,  slightly  by  liquids,  and  not  at  all 
by  gases.  Ex. :  India-rubber,  when  stretched,  tends 
to  fly  back  to  its  original  dimensions.  When  a  solid 
remains  stretched  for  any  length  of  time  it  loses  its 
elasticity.  For  this  reason  a  violin  is  unstrung  when 
not  in  use.  A  drop  of  water  hanging  to  the  nozzle 
of  a  bottle  may  be  touched  by  a  piece  of  glass  and 
drawn  out  to  considerable  length.  "When  let  go,  it 
will  resume  its  spherical  form.  Gases  manifest  no 
tendency  to  return  to  their  original  dimensions  when 
extended. 

Elasticity  of  Torsion  is  the  ten- 
dency of  a  thread  or  wire  which  has 
been  twisted,  to  untwist  again.  It 
is  a  most  delicate  test  of  the 
strength  of  a  force,  and  is  of  great 
service  in  accurate  measurements  in 
physical  science. 

HABDNESS. — A  hard  body  is  one 
which    does    not   readily  yield    to 
pressure.     One  body  is  harder  than  ^ 
another  when  it  will  scratch  or  in- 
dent  it.     This  property  does  not  de-    A  Torsion  Balance. 

2* 


Fig.  4. 


32  NATUEAL  PHILOSOPHY. 

pend  on  density.*  Ex.  :  Gold  is  denser  than  iron, 
yet  is  much  softer.  Mercury  is  a  liquid,  yet  it  is 
twice  as  dense  as  steel. 

BRITTLENESS. — A  brittle  body  is  one  that  is  easily 
broken.  It  is  a  frequent  characteristic  of  hard 
bodies.  Ex.  :  Glass  will  scratch  iron,  and  is  ex- 
tremely brittle. 

*  Density  indicates  the  quantity  of  matter  contained  in  a  given 
bulk.  A  dense  body  has  its  molecules  very  closely  compacted. 
The  word  rare,  which  is  the  opposite  of  dense,  is  generally  ap- 
plied to  gases. 


44  The  smallest  dust  which  floats  upon  the  wind 
Bears  this  strong  impress  of  the  Eternal  mind: 
In  mystery  round  it  subtle  forces  roll, 
And  gravitation  binds  and  guides  the  whole." 


MOLECULAR  FORCES. 

MOLECULAR  FOECES  exist  in  the  molecules  of  mat- 
ter, and  act  only  at  insensible  distances. 

If  we  take  a  piece  of  iron  and  attempt  to  pull  it 
to  pieces,  we  find  that  there  is  a  force  which  holds 
the  particles  together  and  resists  our  efforts.  If  we 
try  to  compress  the  metal,  we  find  that  though  there 
are  pores  in  it  and  the  molecules  do  not  touch  each 
other,  yet  there  is  a  force  which  holds  the  particles 
apart  and  resists  our  efforts  as  before.  If  we  apply 
heat,  the  iron  expands  and  finally  melts.  If,  in 
like  manner,  we  heat  a  bit  of  ice,  we  notice  that  the 
attractive  force  is  gradually  overcome,  the  solid  be- 
comes a  liquid,  and  finally  the  repulsive  force  pre- 
dominates and  the  liquid  passes  off  in  vapor.  In 
turn,  we  can  cool  the  vapor,  and  convert  it  back 
again  into  water  and  ice.  We  thus  see  that  there 
are  two  opposing  forces  which  reside  in  molecules — 
an  Attractive  and  a  Repulsive  force,  and  that  the 
latter  is  Heat.  There  are  three  kinds  of  the  former, 
Cohesion,  Adhesion,  and  Chemical  Affinity* 

*  Chemical  affinity  produces  chemical  changes,  and  its  consid- 
eration belongs  entirely  to  chemistry.  It  binds  together  atoms  or 
molecules  of  different  kinds,  and  causes  them  to  form  new  com- 
pounds. 


2 6  NATURAL  PHILOSOPHY. 

COHESION  is  the  force  which  holds  together  mole- 
cules of  the  same  kind. 

The  three  states  of  Matter. — Matter  is  found  in 
three  states,  solid,  liquid,  and  gaseous.  These  de- 
pend on  the  relation  of  the  Attractive  and  Repulsive 
forces — Cohesion  and  Heat.  If  the  attractive  force 
is  the  stronger,  the  body  is  solid  ;  if  they  are  nearly 
balanced,  it  is  liquid :  if  the  repulsive  force  is 
stronger,  it  is  gaseous.  Most  bodies  may  be  made  to 
take  these  three  states  successively.  Thus,  by  the 
addition  of  heat,  ice  may  be  converted  into  water 
and  thence  into  vapor,  or  vice  versa,  by  the  subtrac- 
tion of  heat.  Most  solids  pass  easily  to  the  liquid 
form,  others  go  directly  from  the  solid  to  the  gas- 
eous state. 

Cohesion  acts  at  insensible  distances. — Take  two  bul- 
lets, and  having  flattened  and  cleaned  one  side  of 
each,  press  them  together  with  a  slightly  twisting 
motion.  They  will  be  found  to  cohere  when  the 
molecules  are  crowded  into  apparent  contact.  Two 
panes  of  pfate-glass,  which  accidentally  fall  against 
each  other,  are  thus  brought  within  the  range  of  the 
attraction  of  Cohesion,  and  are  frequently  cut  and 
polished  as  one  pane.  If  two  globules  of  mercury 
be  brought  near  each  other,  they  remain  separate 
until  the  instant  they  seem  to  touch,  when  they  im- 
mediately coalesce.  Two  freshly-cut  surfaces  of 
rubber,  when  slightly  warmed  and  pressed  together, 
cohere  as  if  they  formed  but  one  piece. 

Welding. — This  process  illustrates  the  principle  just 


COHESION. 


37 


named.  A  rod  of  iron  being  broken,  we  wish  to  mend 
it.  So  we  bring  the  iron  to  a  white-heat  at  the  ends 
which  we  intend  to  unite.  This  partly  overcomes 
the  attraction  of  Cohesion,  and  the  molecules  will 
move  easily  upon  one  another.  Laying  now  the  two 
heated  ends  upon  each  other,  we  pound  them  with  a 
heavy  hammer  until  over  both  surfaces  the  molecules 
are  brought  near  enough  for  the  attraction  of  Co- 
hesion to  bind  them  together.  Iron  and  platinum 
are  the  only  metals  which  can  be  welded,  as  they 
are  the  only  ones  which  become  softened  just  before 
melting.  The  same  property  is  possessed  in  a  re- 
markable degree  by  glass.  Gutta-percha,  when 
warmed  in  water,  can  also  be  welded.  Dough,  wax, 
and  butter  can  be  readily  united  at  common  tem- 
peratures. 

Liquids  tend  to  collect  in  spheres. — Mix  water  and 
alcohol  in  such  proportions  that  a  drop  of  sweet-oil 
will  fall  just  to  the  centre  of  the  fluid.  In  this  way 
the  attraction  of  the  earth  is  neutralized,  and  the 
molecules  of  the  oil  are  left  free  to  arrange  them- 
selves as  they  please.  The  drop  will  form  a  perfect 
sphere.  The  same  tendency  is  seen  in  dew-drops, 
rain-drops,  globules  of  quicksilver,  in  the  manufac- 
ture of  shot,  etc.*  The  reason  for  this  is  simply 
that  the  force  of  Cohesion  acts  toward  the  centre  of 
the  drop.  In  the  spherical  body,  every  portion  of 
the  surface  is  equally  distant  from  the  centre ;  and 
when  that  form  is  assumed  every  molecule  on  the 

*  See  Fourteen  Weeks  in  Chemistry,  Rev.  Ed.,  p.  174. 


3  S  ^4  TURAL  PHIL  OSOPHT. 

outside  is  equally  attracted,  and  there  is  an  equilib- 
rium established. 

Solids  tend  to  form  regular  crystals. — When  a  liquid 
becomes  a  solid,  the  general  tendency  is  to  assume 
a  symmetrical  form.  The  attraction  of  Cohesion 
strives  to  arrange  the  molecules  in  an  orderly  man- 
ner. Each  kind  of  matter  has  its  peculiar  shape 
and  angle,  by  which  its  crystals  may  be  recognized. 
Even  when  different  substances  are  contained  in  the 
same  solution,  they  separate  on  crystallization.  The 
beautiful  finish  and  perfection  of  the  crystals  thus 
formed  in  nature  infinitely  transcend  the  workman- 
ship of  the  highest  art.  God  delights  in  order  as  in 
beauty.  Down  in  the  dark  recesses  of  the  earth  He 
has  fashioned,  by  the  slow  processes  of  His  laws, 
the  rarest  gems — amethysts,  rubies,  and  diamonds. 
There  are  mountain  masses  transparent  as  glass, 
caves  hung  with  stalactites,  crevices  rich  with  gold 
and  silver,  and  lined  with  quartz.  Everywhere  we 
find  regularity  and  symmetry.  This  tendency  is  seen 
in  the  beautiful  crystalline  forms  of  snow-flakes  and 
of  frost.  A  mass  of  ice  seems  irregular,  yet  if  closely 
examined  it  reveals  the  perfect  crystals  crowded  to- 
gether by  the  rapidity  with  which  the  solidification 
took  place.  If  we  watch  the  surface  of  water  which 
is  slowly  freezing,  we  can  see  the  regular  arrange- 
ment of  the  long  crystals  as  they  shoot  out  from 
each  side  of  the  vessel.  The  very  soil  is  largely 
composed  of  broken  and  decomposed  crystals  worn 
down  from  the  rocks  by  the  action  of  the  rain  and  frost. 


COHESION.  ^g 

Illustrations. — "We  can  illustrate  the  formation  of 
crystals  by  adding  alum  to  hot  water  until  no  more 
will  dissolve;  then,  suspending  strings  across  the 
dish,  setting  it  away  to  cool.  Beautiful  octahedral 
crystals  will  collect  over  the  threads  and  the  sides 
of  the  vessel.  The  slower  the  process  the  larger 
will  be  the  crystals.  To  form  the  massive  crystals 
found  in  nature  has  doubtless  required  centuries. 
The  large  ones  seen  in  the  show-windows  are  made 
by  "feeding"  a  single  small  perfect  crystal  every 
day  with  a  fresh  solution.  Melted  iron  rapidly 
cooled  in  a  mould  has  no  time  to  arrange  its 
crystals  perfectly.  If,  however,  the  iron  be  after- 
ward jarred,  as  when  used  for  heavy  cannon,  the 
axles  of  rail-cars,  etc.,  the  molecules  take  on  the 
crystalline  form  and  the  metal  becomes  brittle. 
On  examining  such  a  piece  of  iron  we  can  see  in  a 
fresh  fracture  the  smooth,  shiny  face  of  the  crystals. 

Tempering  and  annealing  illustrate  a  curious  prop- 
erty of  cohesion.  A  piece  of  iron  is  heated  and  then 
plunged  into  oil  or  water.  It  becomes  hard  and 
brittle.  If,  on  the  contrary,  it  be  heated  and  cooled 
slowly,  it  is  made  tough  and  flexible.  Strangely 
enough,  the  same  process  which  hardens  iron  softens 
copper.  It  is  supposed  that  the  arrangement  of  the 
molecules,  and  the  consequent  strength  of  the  metal, 
depend  on  the  time  occupied  in  cooling.  Steel  is 
tempered  by  heating  white-hot,  then  cooling  quickly, 
and  afterward  re-heating  and  cooling  slowly.  The 
more  it  is  re-heated  the  softer  it  becomes. 


4.  o  NA  TUEA  L  PHIL  OSOPHY. 

A  Prince  Rupert's  Drop,  or  Dutch  tear,  consists 
simply  of  a  tear  of  melted  glass  dropped  into  water, 
and  so  cooled  quickly.  The  outside  forms  in  regu- 
lar crystals,  while  the  inner  portion,  not  having  room 
to  expand,  causes  a  violent  strain  upon  the  exterior. 
The  outer  shell  is  strong  enough  to  resist  quite 
a  heavy  blow  with  a  hammer,  but  if  the  small  end 
be  nipped  off,  the  whole  mass  flies  into  powder  with 
a  sharp  explosion. 

^Practical  Questions.— -1.  Why  can  we  not  weld  a  piece  oi  copper  to  one 
of  iron?  2.  WThy  is  a  bar  of  iron  "stronger  than  one  of  wood?  3.  Why  is  a 
piece  of  iron,  when  perfectly  welded,  stronger  than  before  it  was  broken  ?  4. 
Why  do  drops  of  different  liquids  vary  in  size  ?  5.  When  you  drop  medicine, 
why  will  the  last  few  drops  contained  in  the  bottle  be  of  a  larger  size  than  the 
others  ?  6.  Why  are  the  drops  larger  if  you  drop  them  slowly  ?  7.  Why  is  a 
tube  stronger  than  a  rod  of  the  same  weight  ?  8 .  Why,  if  you  melt  scraps  of 
lead,  will  they  form  a  solid  mass  when  cooled  ?  9.  In  what  liquids  is  the  force 
of  cohesion  greatest  ?  10.  Name  some  solids  which  will  volatilize  without 
melting. 

ADHESION  is  the  force  which  holds  together  mole- 
cules of  different  kinds.  Ex. :  We  fasten  together 
two  pieces  of  wood  with  glue,  two  pieces  of  china 
with  cement,  two  bricks  with  mortar,  two  sheets  of 
paper  with  mucilage,  two  pieces  of  tin  with  solder, 
glass  and  wood  with  putty,  glass  and  brass  with 
plaster  of  Paris,  and  paper  to  the  wall  with  paste. 
Paint  adheres  to  the  wood-work,  dust  to  the  wall, 
and  chalk  to  the  blackboard. 

The  adhesion  between  animal  charcoal  and  va- 
rious coloring  matters  is  very  great.  If  any  liquid 
containing  these  substances  be  filtered  through 
it,  the  foreign  matter  in  solution  will  adhere  to 
the  charcoal,  while  the  liquid  will  run  through 


ADHESION. 


by 

13 


Fig.  5. 


perfectly  colorless.  Syrup  is  thus  purified 
passing  through  a  layer  of  charcoal  12  or 
feet  thick.  The  cleansing  qualities  of  common 
charcoal  in  water-filters  is  probably  due  largely  to 
this  property.  Bubbles  can  be 
blown  from  soapsuds,  because 
the  soap  by  its  adhesive  force 
holds  the  particles  of  water  to- 
gether.* 

Capillary  Attraction  (capillus, 
a  hair)  is  a  variety  of  adhesion. 

It    may    be    seen    when    two    

plates   of   glass  are  placed  in 

water,  as  shown  in  Fig.  5,  but 

is   exhibited   most   strikingly  in  very          Fi§-6- 

fine  tubes,  whence  the  name.f 

1.  If  we  insert  a  small  glass  tube 
in    water,    the    liquid    will    rise    in 
the  tube.     The  smaller  the  tube,  the 
greater  will  be  the  height.     In  this 
case,   it   is  evident    that    the    adhe- 
sive attraction  of  the  glass  is  greater 
than  the    cohesive    attraction    of 
water.      There  is  an  attraction  between   the   glass 
and  water. 

*  If  a  bubble  be  blown  at  the  end  of  a  glass  tube,  the  thin  film 
of  water  contracting  by  its  cohesive  force  will  frequently  drive 
back  the  air  through  the  pipe  with  sufficient  strength  to  extin- 
guish the  flame  of  a  candle. 

f  These  tubes  may  be  easily  drawn  to  any  length  and  size,  from 
French-glass  tubing,  in  the  heat  of  a  common  alcohol-lamp. 


42  NATURAL  PHILOSOPHY.- 

2.  If  we  insert  a  glass  tube  in  a  dish  of  mercury, 
the  capillary,  action  is  reversed  and  the  height  of  the 
Fig.  7.  liquid  is  less  than  the  general  level. 

In  this  case  the  adhesive  attraction  of 
the  glass  is  less  than  the  cohesive  at- 
traction of  the  mercury.  There  is  an 
apparent  repulsion  between  the  glass 
and  mercury. 

Illustrations. — 1.  The  wick  of  an  oil- 
lamp  or  a  candle  is  a  bundle  of  fine 
capillary  tubes  or  pores  which  elevate 
the  oil  or  melted  fat  and  feed  the  flame.  Thus  ex- 
tinguishers are  needed  to  an  alcohol-lamp,  because 
by  capillary  attraction  the  liquid  tends  to  rise  to  the 
top  and  there  evaporate  until  the  lamp  is  emptied. 

2.  If  the  end  of  a  towel  be  dipped  in  a  basin  of 
water,  the  whole  towel  will  soon  be  wet  by  capillary 
action  through  the  fine  pores  and  tubes  of  the  cloth. 
Thus  also  the  capillary  tubes  of  a  towel  dry  one's 
face  after  washing. 

3.  Blotting-paper   absorbs  ink  by  means   of  its 
capillary  tubes. 

4.  Water  poured  in  the  saucer  of  a  flower-pot  is 
elevated   through   the   pores   of   the   earth   to   the 
plant. 

5.  By  means  of  the  capillary  force  water  is  drawn 
up  through  the  earth  to  the  surface  of  the  ground,  and 
there  moistens  the  roots  of  plants  and  supplies  them 
with  the  materials  of  growth.    In  the  winter,  when  the 
surface  is  frozen,  the  water  still  finds  its  way  upward, 


ADHESION. 


43 


freezing  into  ice,  which  on  melting  in  the  spring 
produces  mud,  even  where  there  has  been  but  little 
rain  or  snow.  Ploughing  ground  causes  it  to  endure 
drought  better,  because  it  stirs  the  soil  and  increases 
the  size  of  the  capillary  pores,  thus  partially  pre- 
venting the  water  from  being  carried  to  the  surface 
and  there  evaporated. 

6.  Ropes  absorb  water  by  capillary  action,  swell, 
and   are  shortened.     Clothes-lines  are  thus   tight- 
ened and  sometimes  broken  in  a  shower.     A  rope 
will  shrink  with  such  force  as  to  lift  a  great  weight.* 

7.  Houses  are  rendered  damp  by  moisture  drawn 
in  by  the  capillary  action  of  the  pores  in  the  wood 
or  stone  walls. 

8.  Millstones  in  Germany  are  split  off  by  means 
of   wooden  wedges.     These  being   driven   in  when 
dry,  afterward  absorb  moisture,  swell,  and  burst  the 

*  A  curious  illustration  of  this  is  given  in  the  following  story. 
When  the  great  Egyptian  obelisk  was  to  be  raised  in  the  square 
of  St.  Peter's,  at  Rome,  Pope  Sixtus  V.  proclaimed  that  no  one 
should  utter  a  word  aloud  until  the  engineer  announced  that  all 
danger  was  passed.  As  the  majestic  column  ascends,  all  eyes 
watch  it  with  wonder  and  awe.  Slowly  it  rises,  inch  by  inch, 
foot  by  foot,  until  the  task  is  almost  completed,  when  the  strain 
becomes  too  great.  The  huge  ropes  yield  and  slip.  The  workmen 
are  dismayed  and  fly  wildly  to  escape  the  impending  mass  of  stone. 
Suddenly  a  voice  breaks  the  silence.  "  Wet  tlie  ropes"  rings  out 
clear-toned  as  a  trumpet.  The  crowd  look.  There,  on  a  high 
post,  standing  on  tiptoe,  his  eyes  glittering  with  the  intensity  of 
excitement,  is  the  architect  Zapaglia.  His  voice  and  appearance 
startle  every  one,  but  his  words  inspire.  He  is  obeyed.  The 
ropes  swell  and  bite  into  the  stone.  The  column  ascends  again, 
and  in  a  moment  more  stands  securely  on  its  pedestal. 


44  NATURAL  PHILOSOPHY. 

rock,  thus  saving  an  immense  expenditure  of  time 
and  money. 

Solution. — If  we  put  a  little  sugar  in  water,  it 
will  dissolve  because  the  adhesive  force  of  the  water 
is  stronger  than  the  cohesive  force  of  the  sugar.  As 
heat  weakens  the  cohesive  force,  it  commonly 
hastens  solution ;  and  we  can  dissolve  more  of  a 
substance,  and  more  rapidly,  in  hot  water  than  in 
cold.  In  like  manner  .pulverizing  a  solid  hastens 
its  solution.  A  solid  will  not  dissolve  in  a  liquid 
if  there  is  no  adhesion  between  them.  Water 
absorbs  great  quantities  of  the  various  gases  by 
means  of  adhesion.  It  always  contains  air,  which 
renders  it  pleasant  to  the  taste.  In  simply  pour- 
ing it  from  one  dish  to  another,  we  notice  that 
bubbles  of  air  adhering  to  the  stream  are  carried 
down,  and  then  rise  to  the  surface  and  break.  It 
has  been  proposed  to  apply  this  principle  to  the 
ventilation  of  mines.  As  both  pressure  and  cold 
weaken  the  repulsive  force  of  the  gases,  they  favor 
the  adhesion  between  the  molecules  of  the  gases 
and  those  of  water.  Soda-water  receives  its  effer- 
vescence and  pungent  taste  from  carbonic  acid 
gas,  which,  being  absorbed  under  great  pressure, 
escapes  in  little  sparkling  bubbles  as  soon  as  the 
pressure  is  removed. 

Diffusion  of  liquids. — Let  a  tall  jar  be  partly  filled 
with  water  colored  by  blue  litmus.  Then,  by  means 
of  a  long  funnel-tube,  pour  a  little  clear  water  con- 
taining a  few  drops  of  oil  of  vitriol  to  the  bottom,  be- 


ADHESION. 

neath  the  colored  water.     The  two  Fig._8. 

will  be  distinctly  defined  at  first, 
but  in  a  few  days  they  will  mix 
throughout,  as  will  be  seen  by  the 
change  of  color  from  blue  to  red. 
A  drop  of  oil  of  vitriol  may  thus 
be  distributed  through  a  quart  of 
water.  Most  liquids  will  mingle 
when  brought  in  contact.  If,  how- 
ever, there  be  no  adhesion  between 
their  molecules,  they  will  not  mix,, 
and  will  even  separate  when  thor- 
oughly shaken  together. 

Diffusion  of  gases. — Hydrogen  gas  is  only  Fig 
T\-  as  heavy  as  common  air.  Yet,  if  two 
bottles  be  arranged  as  in  the  figure,  the 
lower  one  filled  with  the  heavy  gas  and 
the  upper  with  the  lighter,  the  gases  will 
soon  be  found  thoroughly  mingled. 

Osmose  of  liquids. — When  liquids  are  sepa- 
rated from  each  other  by  a  thin  porous 
substance,  they  do  not  mingle  uniformly, 
but  the  interchange  is  modified  in  a  most 
curious  manner,  according  to  the  nature  of 
the  liquid  and  the  substance  used.  At  the 
end  of  a  glass  tube,  as  in  Fig.  10,  fasten  a  bladder 
full  of  alcohol.  Fill  the  jar  with  water,  and  mark 
the  height  to  which  the  alcohol  ascends  in  the  tube. 
The  column  will  soon  be  found  to  be  gradually 
rising.  On  examination  we  shall  see  that  the  alco- 


NATURAL  PHILOSOPHY. 


liol  lias  been  passing  out 
through  the  pores  of  the 
bladder  and  mixing  with 
the  water,  while  the  water 
has  been  coming  in  more 
rapidly.  This  has  been 
explained  by  supposing 
that  the  water  adheres 
more  strongly  than  the 
alcohol  to  the  bladder. 
Thus,  by  capillary  attrac- 
tion it  is  drawn  through 
the  membrane,  and  on 
the  inner  side,  by  the 
law  of  diffusion  of  liquids, 
mingles  with  the  alcohol. 
By  a  similar  process  some 
alcohol  passes  outward  and  mixes  with  the  water. 
Whatever  liquids  are  used,  that  one  which  wets 
the  membrane  most  readily  will  pass  through 
most  rapidly.  If  we  should  use  a  collodion  bal- 
loon, instead  of  a  bladder,  the  effect  would  be 
reversed. 

Osmose  of  gases. — The  following  experiment  would 
seem  to  render  it  probable  that  there  is  a  similar 
osmose  of  gases.  Fit  a  small  porous  cup,  such 
as  is  used  with  Grove's  Battery,  with  a  cork  and 
glass  tube,  as  shown  in  Fig.  11.  Fasten  the  tube 
so  that  it  will  just  dip  below  the  surface  of  the 
water  in  the  lower  jar.  Now  invert  over  the  porous 


ADHESION. 


Fig.  11. 


cup  a  receiver  of  hydrogen 
gas.  This  gas  will  pass 
through  the  pores  of  the  cup 
and  down  the  tube  so  rapidly 
as  to  bubble  up  through  the 
water  almost  instantly. 

Rose  balloons,  so  popular 
as  toys,  soon  lose  their 
buoyancy,  because  the  hy- 
drogen escapes  through  the 
pores  of  the  rubber  much 
more  rapidly  than  the  air 
comes  in  to  take  its  place. 
The  balloon  soon  shrinks 
and  drops  down  from  the 
weight  of  the  rubber. 


^Practical  Questions.— \.  Why  does  cloth  shrink  when  wet?  2.  Why 
do  sailors  at  a  boat-race  wet  the  sails  ?  3.  Why  does  not  writing-paper  blot  ? 
4.  Why  does  paint  prevent  wood  from  shrinking?  5.  What  is  the  shape  of 
the  surface  of  a  glass  of  water  ?  One  of  mercury?  6.  Why  can  we  not  dry 
a  towel  perfectly  by  wringing?  7.  Why  will  not  water  run  through  a  fine 
sieve  when  the  wires  have  been  greased  ?  8.  Why  will  camphor  dissolve  in 
alcohol  and  not  in  water  ?  9.  Why  will  mercury  rise  in  zinc  tubes  as  water 
will  in  glass  tubes?  10.  Why  is  it  so  difficult  to  lift  a  board  out  of  water? 
11.  Why  will  ink  spilled  on  the  edge  of  a  book  extend  farther  inside  than  if 
ppilled  on  the  side  of  the  leaves  ?  12.  If  you  should  happen  to  spill  some  ink 
on  the  edge  of  your  book,  ought  you  to  press  the  leaves  together?  13.  Why 
can  yon  not  mix  water  and  oil?  14.  What  is  the  object  of  the  spout  on  a 
pitcher  ?  Am.— The  water  would  run  down  the  side  of  the  pitcher  by  the 
force  of  Adhesion,  but  the  spout  throws  it  into  the  hands  of  Gravitation  before 
Adhesion  can  catch  it.  15.  Why  will  water  wet  your  hand,  while  mercury 
will  not?  16.  Why  is  a  pail  or  tub  liable  to  fall  to  pieces  if  not  filled  with 
water  or  kept  in  the  cellar  ?  1  7.  Name  instances  where  the  attraction  of 
adhesion  is  stronger  than  that  of  cohesion. 


ATTRACTION  OF  GRAVITATION. 


WE  have  spoken  of  the  attraction  existing  be- 
tween the  molecules  of  bodies  at  minute  distances. 
"We  now  notice  another  form  of  the  same  attrac- 
tion, which  acts  between  masses  at  all  distances. 

GRAND  LAW  OF  GRAVITATION.* — Every  particle  of 

matter  in  the  universe  attracts  every  other  particle 

of  matter  with  a  force  directly  proportional  to  its 

mass,  and  decreasing  as  the  square  of  the  distance 

Fig.  12.  increases. 

Illustrations. — A  stone  falls  to  the 
ground  because  the  earth  attracts 
it ;  but  in  turn  the  stone  attracts 
the  earth.  The  force  of  the  attrac- 
tion is  in  proportion  to  their  rel- 
ative mass.  They  each  move  to 
meet  the  other,  but  the  stone 
passes  through  as  much  greater 
distance  than  the  earth  as  its 
mass  is  less.  A  plumb-line  hang- 
ing near  a  mountain  is  attracted 
out. of  a  perpendicular.  In  the 
c  figure,  A  B  represents  the  ordi- 
nary position  of  the  line,  while  A  C  indicates  the 
attractive  power  (exaggerated)  of  the  mountain. 


*  See  "  Fourteen  Weeks  in  Astronomy,"  p.  34,  for  history  of 
this  law. 


49 

This  law  is  not  confined  to  our  own  world.  By 
it  the  heavenly  bodies  are  bound  to  each  other,  and 
thus  kept  in  their  orbits.  It  may  help  us  to  con- 
ceive how  the  earth  is  supported,  if  we  imagine  the 
sun  letting  down  a  huge  cable,  and  every  star  in 
the  heavens  a  tiny  thread,  to  hold  our  globe  in  its 
place,  while  it  in  turn  sends  back  a  cord  to  every  one. 
So  we  are  bound  to  them  and  they  to  us.  Thus 
the  worlds  throughout  space  are  linked  together  by 
these  cords  of  mutual  attraction,  which,  interweaving 
in  every  direction,  make  the  universe  a  unit. 

GRAVITATION  is  the  general  term  applied  to  the 
attraction  that  exists  between  all  bodies  in  the 
universe.  GRAVITY  is  used  to  designate  the  earth's 
attraction  for  all  terrestrial  bodies ;  it  tends  to  draw 
them  toward  the  centre  of  the  earth.  WEIGHT  is 
the  measure  of  the  force  of  gravity.  When  we 
say  that  a  body  weighs  10  Ibs.,  we  mean  that  the 
earth  attracts  it  that  amount.  The  following  gen- 
eral principles  wiU  explain  the  various  phenomena 
of  weight. 

I.  The  iveight  of  a  body  at  the  centre  of  the  earth 
is  nothing,  because  the  attraction  is  there  equal  in 
every  direction. 

II.  The  weight  of  a  body  above  the  surface  of  tJie 
earth  decreases  as  the  square  of  its  distance  from 
the  centre  of  the  earth  increases.     Ex. :  A  body  at 
the  surface  of  the  earth  (4000  miles  from  the  centre) 
weighs  100  Ibs.     What   would  be  its  weight  1000 
miles  above  the  surface  (5000  miles  from  the  centre) 

3 


£o  NATURAL  PHILOSOPHY. 

of  the  earth?     Solution— (5,000  m.)2  :  (4,000  m.)?-  :: 
100  Ibs.  :x=Q4:  Ibs. 

III.  The  weight  of  a  body  varies  on  different  por~ 
tions  of  the  surface  of  the  earth*  It  will  be  least  at 
the  equator,  (1)  because,  on  account  of  the  bulging 
form  of  our  globe,  a  body  is  there  pushed  out 
from  the  mass  of  the  earth,  and  so  removed  from 
the  centre  of  attraction  ;  (2)  because  the  centrifugal 
force  is  there  the  strongest.  It  will  be  greatest  at 
the  poles,  (1)  because,  on  account  of  the  flattening 
of  the  earth,  a  body  is  there  brought  nearer  its 
mass  and  the  centre  of  attraction;  (2)  because 
there  is  no  centrifugal  force  at  those  points. 

FALLING  BODIES. — Since  the  attraction  of  the 
earth  is  toward  its  centre,  all  bodies  falling  freely 
move  in  a  direct  line  toward  that  point.  This 
line  is,  called  a  vertical  or  plumb  line  (Plumbum, 
lead,  because  a  lead  weight  is  generally  used  by 
mechanics).  All  plumb-lines  point  toward  the  cen- 
tre of  the  earth. 

Laws  of  Falling  Bodies. — I.  Under  the  influence  of 
gravity  alone,  all  bodies  fall  with  equal  rapidity. 

This  is  well  illustrated  by  the  "  Guinea  and 
feather  experiment."  Let  a  coin  and  a  feather  be 
placed  in  a  long  tube,  as  in  Fig.  13,  and  the  air 
exhausted.  Quickly  invert  the  tube,  and  the  two 

*  In  these  statements  concerning  weight,  a  spring-scale  is  sup- 
posed to  be  employed.  With  a  pair  of  balances,  the  weights  used 
would  become  heavier  or  lighter  in  the  same  proportion  as  the 
body  to  be  weighed. 


GRAVITATION. 


51 


bodies  will  fall  in  the  same  time.     Let  in  the  air 
again,    and    now   the    feather  Fig.  13, 

will  come  fluttering  down  long 
after  the  coin  has  reached  the 
bottom.  Hence  we  conclude 
that  in  a  vacuum  all  bodies 
descend  with  equal  velocity, 
and  that  the  resistance  of 
the  air  is  the  cause  of  the 
variation  we  see  in  the  falling 
of  light  and  heavy  bodies. 
The  same  fact  may  be  noticed 
in  the  case  of  a  sheet  of  paper. 
When  spread  out,  it  merely 
flutters  to  the  ground;  but 
when  rolled  together  in  a  com- 
pact mass,  it  falls  like  lead. 
In  this  case  we  have  not  in- 
creased the  force  of  attraction, 
but  we  have  decreased  the 
resistance  of  the  air. 

II.  In  the  first  second,  a  body  gains  a  velocity  of  32 
feet  and  falls  16  feet. — This  has  been  proved  by  ex- 
perimeais  with  the  pendulum,  and  with  Atwood's 
machine,  an  instrument  constructed  with  great  ac- 
curacy for  such  investigations.  It  will  be  noticed 
that  16  feet,  the  distance  passed  through  the  first 
second,  is  the  mean  between  0,  the  velocity  at  the 
beginning,  and  32,  the  velocity  at  the  close  of  the 
second. 


5  2  NA  TURAL  PHIL  OSOPHY. 

III.  In  any  succeeding  second,  the  velocity  is  16  feet 
multiplied  by  the  corresponding  even  number  4,  6,  8, 
etc.;  and  the  distance  is  16  feet  multiplied  by  the  corre- 
sponding odd  number*  3,  5,  7,  9,  etc. 

1.  The  body  commences  the  second  second  with 
a  velocity  of  32  feet,  and  as  gravity  is  a  constant 
force,  gains  32  feet  more  during  the  second,  making 
64  feet =4x16  feet.  It  commences  the  third  sec- 
ond with  a  velocity  of  64  feet,  and  gains  32  feet 
more;  making  96  feet— 6x16  feet.  2.  The  mean 
between  32  feet,  the  velocity  at  the  beginning  of 
the  second  second  and  64  feet,  the  velocity  at  the 
close,  is  48  feet=3x!6  feet.  The  mean  between 
64  feet,  the  velocity  at  the  beginning  of  the  third 
second  and  96  feet,  the  velocity  at  the  close,  is  80 
feet— 5x16  feet.  Hence  we  conclude  that  the 
velocities  are  as  the  even  numbers,  and  the  dis- 
tances as  the  odd  numbers. 

IY.  In  any  number  of  seconds,  a  body  falls  16  feet 
multiplied  by  the  square  of  the  number  of  seconds. 

"We  have  just  seen  that  a  body  falls  16  feet  the 
first  second,  and  48  feet  the  second.  Hence  in  two 
seconds  it  falls  16  feet+48  feet=64  feet=23x!6 
feet.  In  three  seconds  it  falls  16+48  +  80  feet= 
144feet=32xl6  feet. 

Equations  of  falling  bodies. — If  we   represent   the 

*  The  odd  number  corresponding  to  any  second  is  easily  found 
by  doubling  the  number  of  the  second  and  subtracting  1  from 
the  result.  Ex. :  Required  the  odd  number  for  the  eighth  second. 
8  X  2=16.  16  -1=15,  the  eighth  odd  number. 


GEAV1TATION.  ^ 

velocity  of  a  falling  body  by  v,  the  distance  by  d, 
and  the  time  by  £,  the  following  equations  can  be 
derived  from  the  foregoing  laws. 

V  —  32t  ^,   .   .   (1). 
d  =  16f  ,  .   .   .   (2). 
.   .   (3). 


If,  now,  we  let  g  represent  the  constant  force  of 
gravity,  16  feet  in  each  second,  we  have  from  the  (3) 

v  =  2Vgd  ....  (4). 


Easy  way  of  finding  the  depth  of  a  ivell.  —  Let  -a 
stone  fall  into  it,  and,  with  a  watch  or  by  the  beat 
of  the  pulse,  count  the  seconds  that  elapse  before 
you  hear  it  strike  the  bottom.  Square  the  number 
of  seconds,  and  multiply  16  feet  by  the  result.  The 
product  is  the  depth.  A  little  time  is  required  for 
the  sound  to  come  to  the  ear,  but  this  is  so  slight 
that  it  may  be  neglected. 

When  a  body  is  thrown  upward  the  same  principles 
apply,  only  in  a  reverse  manner.  Through  the  in- 
fluence of  gravity  it  loses  32  feet  in  velocity  each 
second  it  rises.  The  .velocity  necessary  in  order  to 
elevate  It  to  a  certain  point,  must  be  that  which  it 
would  acquire  in  falling  that  distance.  It  will  rise 
just  as  high  in  a  given  time  as  it  would  fall  in  the 
same  time.  If  a  ball  be  thrown  vertically  into  the 
air,  it  will  be  as  long  in  falling  as  in  rising.  In 
theory,  it  will  strike  the  earth  with  the  same  force 
with  which  it  was  thrown  :  in  practice,  however,  the 


54 


NATURAL  PHILOSOPHY. 


ball  loses  about  -J-  of  its  force  in  rising  and  an  equal 
amount  in  falling,  owing  to  the  resistance  of  the  air. 
CENTRE  OF  GRAVITY. — This  is  that  point  on  which, 
if  supported,  a  body  will  balance  itself.     The   line 
of  direction  is  a  vertical  line  drawn  from  the  centre 
of  gravity.     It  is  the  line  along  which  the  centre  of 
gravity  would  pass,  if  the  body  should  fall.     When 
Fig.  11  a  body  is  at  rest,  the  force  of  gravity 

which  attracts  it  is   said   to   be   in 
equilibrium. 

The  three  states  of  equilibrium  are 
ftdble,  unstable,  and  indifferent. ' 

I.  A  body  is  in  stable  equilibrium 
when  the  centre  of  gravity  is  below 
the  point  of  support,  or  when  any 
movement  tends  to  raise  the  centre 
of  gravity.     In  Fig.  14  a  man  has 
the  centre  of  gravity  lowered  below 
the  point  of  support  by  means  of  lead 
balls.      Remove    these 
and     he     immediately 
falls,  but  with  them  he 
is    in     stable    equilib- 
rium.     Any  movement 
tends  to  raise  the  cen- 
tre of  gravity,   and  he 
returns    quickly    to    a 
state  of  rest.      The  toy 
in  Fig.  15  illustrates  a 
paradox  in  philosophy, 


GRAVITATION. 


55 


viz. :   "  When  a  body  tends  to  fall,  hang  a  weight 
on  the  heavy  side  to  steady  it."     A          rig.  IG. 
needle   may  be  easily  balanced  on 
its  point  by  means  of  a  cork  and 
two    jack-knives    (Fig.    16),    which 
lower  its  centre  of  gravity. 

II.  A  body  is  said  to  be  in  un- 
stable equilibrium,  when  the  centre 

of  gravity  is  above  the  point  of  support,  or  when 
any  movement  tends  to  lower  the  centre  of  gravity. 
If  we  take  the  cork,  as  balanced  in  Fig.  16,  and 
invert  it,  we  shall  find  it  very  difficult  to  balance 
the  needle  ;  and,  if  we  succeed,  it  will  readily  topple 
off,  because  the  least  motion  tends  to  lower  the 
centre  of  gravity.  Fig.  17. 

III.  A  body  is  said  to  be  in 
different  equilibrium  when  the  < 
tre  of  gravity  is  at  the  point 
support,  or  when  any  move- 
ment tends  neither  to  ele- 
vate nor  lower  the  centre  of 
gravity.     A  ball  of  uniform 
density  on  a  level  surface 

will  come  to  rest  in  any 
position,  because  the  cen-  n 
tre  of  gravity  moves  in  a 
line  parallel  to  the  floor. 

The  centre  of  gravity  may 
be  found  either  by  balancing 
the  body  or  by  suspending  it  from  one  corner,  as  in 


5  6  NA  TURAL  PHIL OSOPH Y. 

Fig.  17.  By  means  of  a  plumb-line,  obtain  the 
line  of  direction,  A  E ;  then  hang  it  the  same  way 
from  another  corner,  and  mark  the  line  of  direction, 
B  D.  The  point  C,  where  the  two  lines  cross,  is 
the  centre  of  gravity. 

The  following  general  principles  will  be   readily- 
apparent. 

a.  The  centre  of  gravity  in  a  body  always  tends 
to  seek  the  lowest  point. 

b.  A  body  will  never  tip  over   while   the  line  of 
direction   falls   within  the  base,  but  will  do  so  as 
soon  as  it  falls  without. 

c.  The   higher   the   centre    of    gravity   must    be 
raised  before  the  line  of  direction  will  fall  outside 
of  the  base,  the  firmer  a  body  stands. 

d.  The  lower  the  centre  of  gravity  lies  in  a  body, 
the  more  stable  it  is. 

e.  In  general,  narrowness  of  base  combined  with 
height  tends  to  instability;  while  breadth  of  base 
and    lowness     produce    stability.     The    celebrated 
leaning  tower  of  Pisa,  in  Italy,  illustrates  the  prin- 
ciples of  gravity.     It  is  about  180  feet  high,  and  its 
top  leans  15  feet,  yet  the  line  of  direction  falls  so 
far  within  the  base  that  it  is  perfectly  stable,  as  it 
has  stood  for  seven  centuries.     The  feeling  experi- 
enced by  a  person  who  for  the  first  time  looks  down 
from  the  lower  side  of  this  apparently  impending 
structure,  is  startling  indeed.    The  towers  of  Bologna 
(Fig.  18)  are  also  very  wonderful.    The  lower  of  these 
is  130  feet  high  and  is  inclined  eight  feet  from  the 
perpendicular. 


GRAVITATION. 
Fig.  18. 


57 


Leaning  Tower  at  Bologna. 

Physiological  fads. — Our  feet  and  the  space  be- 
tween them  form  the  base  on  which  we  stand.  By 
turning  our  toes  outward  we  increase  its  breadth. 
When  we  stand  on  one  foot,  we  bend  over  so  as  to 
bring  the  line  of  direction  within  this  narrow  base. 
When  we  carry  a  pail  of  water,  we  balance  it  by 
leaning  in  the  opposite  direction.  When  we  walk 
up  hill  we  lean  forward,  and  in  going  down  hill  we 
incline  backward,  in  unconscious  obedience  to  the 

3* 


NATURAL  PHILOSOPHY. 


Fig.  19. 


laws  of  gravity.  "We  bend  forward  when  we  wish,  to 
rise  from  a  chair,  in  order  to  bring  the  centre  of 
gravity  over .  our  feet ;  our  muscles  not  having  suf- 
ficient strength  to  raise  our  bodies  without  this  aid. 
When  we  walk,  we  lean  forward,  so  as  to  bring  the 
centre  of  gravity  as  far  in  front  as  possible.  Thus, 
walking  is  a  process  of  falling.  When  we  run,  we 
lean  further  forward,  and  so  fall  faster.  (Phys.,  p.  49.) 
THE  PENDULUM  consists  of  a  weight  so  suspended 
as  to  swing  freely.  Its  movements  to  and  fro  are 
termed  vibrations  or  oscillations.  The  path  through 
which  it  passes  is  called  the  arc.  The  extent  to 

which  it  goes  in  either  di- 
rection is  styled  its  ampli- 
tude. Vibrations  performed 
in  equal  times  are  termed 
isochronous  (isos,  equal,  and 
cJironos,  time). 

1st  Law. — In  the  same 
pendulum,  all  vibrations  of 
small  amplitude  are  iso- 
chronous. If  we  let  one  of 
the  balls  represented  in 
Fig.  19  swing  through  a 
short  arc,  and  count  the 
•number  of  oscillations  per 
minute,  we  shall  find  them 
uniform.  This  property 
of  the  pendulum  was  dis- 
covered by  Galileo  when  a 


GRAVITATION. 


59 


boy,  while  sitting  in  the  cathedral  at  Pisa  and  watch- 
ing the  vibrations  of  a  bronze  chandelier  which  hung 
from  the  ceiling.  Others  had  seen  this  before  him. 
He  first  noticed  that  the  swinging  lamps  measured 
time  as  well  as  shed  light. 

2d  Law. — The  time  of  vibration  is  not  affected 
by  the  material  of  which  the  weight  is  composed. 
In  Fig.  19,  let  D  be  a  ball  of  iron,  and  C  one  of  wood. 
They  will  be  found  to  oscillate  together. 

3d  Law. — The  times  of  the  vibrations  of  different 
pendulums  are  proportional  to  the  square  roots  of 
their  respective  lengths.  Let  A  be  |  the  length  of 
C,  and  it  will  vibrate  three  times  as  fast.  If  B  be 
|  the  length  of  C,  it  will  vibrate  twice  as  fast. 
Conversely,  the  lengths  of  different  pendulums  are 
proportional  to  the  squares  of  their  Fig.  20. 

times  of  vibration.  A  pendulum 
which  vibrates  seconds  must  be  four 
times  as  long  as  one  which  vibrates 
half-seconds,  and  sixteen  times  as 
long  as  one  which  vibrates  quarter- 
seconds.  The  apparatus  represented 
in  Fig.  20,  can  be  used  to  illustrate 
very  clearly  the  preceding  laws. 

4J.li  Law. — The  time  of  the  vibra- 
tion of  the  same  pendulum  will  vary 
at  different  places  on  the  earth.  It 
will  decrease  as  the  square  root  of 
the  force  of  gravity  increases.  At 
the  equator  a  pendulum  vibrates 


<5o 


NATURAL  PHILOSOPHY. 


Fig.  21. 


most  slowly,  and  at  the  poles  most  rapidly.  The 
length  of  a  second-pendulum  at  New  York  is  39^ 
inches. 

Centre  of  oscillation. — The  length  of  a  pendulum  is 
not  its  absolute  length  as  measured  from  one  ex- 
tremity to  the  other,  but  the  distance  from  the 
point  of  support  to  the  centre  of  oscillation.  The 
upper  part  tends  to  move  faster  than  the  lower  part, 
and  so  hastens  the  speed  of  the  pendulum.  The 
lower  part  tends  to  move  slower  than  the  upper  part, 
and  so  retards  the  speed  of  the  pendulum.  Be- 
tween these  two  extremes  is  a  point  which  is  neither 

quickened  nor  imped- 
ed by  the  rest,  but 
moves  in  the  same 
time  that  it  would  if 
it  were  a  particle 
swinging  by  an  imag- 
inary line.  This  point 
is  called  the  centre  of 
oscillation.  It  lies  a 
little  below  the  centre 
of  gravity.  In  Fig.  21 
is  shown  an  apparatus 
containing  pendulums 
of  different  shapes,  but 
all  having  the  same 
absolute  length.  If 
they  are  started  to- 
gether, they  will  im- 


GRAVITATION. 


61 


mediately  diverge,  no  two  vibrat-  Fig.  22. 

ing  in  the  same  time.  As  pendu- 
lums, they  are  not  of  the  same 
length. 

The  centre  of  oscillation  is  found 
by  trial.  It  has  been  discovered 
that  the  point  of  suspension  and 
the  centre  of  oscillation  are  inter- 
changeable. If,  therefore,  a  pen- 
dulum be  inverted,  and  a  point 
found  at  which  it  will  vibrate  in 
the  same  time  as  before,  this  is 
known  to  be  the  centre  of  oscilla- 
tion ;  while  the  old  point  of  sus- 
pension becomes  the  new  centre  of 
oscillation. 

The  Pendulum  as  a  Time-keeper. 
— The  friction  at  the  point  of  sus- 
pension, and  the  resistance  of  the 
air,  soon  destroy  the  motion  of  the 
pendulum  and  bring  it  to  rest.  The 
common  clock  is  simply  a  machine 
for  keeping  up  the  vibration  of  the 
pendulum  and  counting  its  beats.  | 
In  Fig.  22,  E  is  the  scape-wheel 
driven  by  the  force  of  the  clock- 

V  CfcjEB** 

weight  or  spring,  and  m  n  the  es- 
capement, moved  by  the  forked  arm  A  B,  so  that 
only  one  cog  of  the  wheel  can  pass  at  each  double 
vibration    of  the   pendulum.     In   this   manner  the 


62 


NATURAL  PHILOSOPHY. 


oscillations  are  counted  by  the  cogs  on  the  wheel, 
while  the  friction  and  resistance  are  overcome  by 
the  action  of  the  weight  or  spring.*  As  "  heat  ex- 
pands and  cold  contracts,"  a  pendulum  increases  in 
length  in  summer  and  shortens  in  winter.  A  clock, 
Big.  23.  therefore,  loses  time  in  summer  and 
gains  in  winter.  To  regulate  a  com- 
mon clock,  we  raise  or  lower  the  pen- 
dulum-bob, L,  by  means  of  a  nut  v  at 
the  lower  end  of  the  rod. 

The  compensation  or  gridiron  pendu- 
lum, consists  of  several  brass  and 
steel  rods,  which  are  so  connected  that 
the  brass,  h,  k,  will  lengthen  upward 
and  the  steel,  a,  b,  c,  d,  will  lengthen 
downward,  and  thus  the  centre  of  oscil- 
lation will  be  unchanged  by  any  varia- 
tion in  temperature.  The  mercurial 
pendulum  contains  a  cup  of  mercury 
which  expands  upward  while  the  pen- 
.dulum-rod  expands  downward,  and 
thus  keeps  the  centre  of  oscillation 
stationary. 

Various    uses   of  the    Pendulum. — 1. 
Since  the  time  of  the  vibration  of  a  pen- 
dulum indicates  the  force  of  gravity, 
and  since  the  force  of  gravity  decreases  as  the  square 


*  The  action  of  a  clock  is  best  shown  by  procuring  the  works 
of  an  old  clock,  and  watching  the  movements  of  the  various  parts. 


GRAVITATION.  £3 

of  the  distance  from  the  centre  of  the  earth  increases, 
we  may,  in  this  manner,  find  the  semi-diameter  of  the 
earth  at  various  places,  and  thus  ascertain  the  figure 
of  our  globe.  2.  Knowing  the  force  of  gravity  at 
any  point,  the  velocity  of  a  falling  body  can  be 
determined.  3.  It  may  be  used  as  a  standard  of 
measures.  4.  Foucault  devised  an  ingenious  method 
of  showing  the  revolution  of  the  earth  on  its  axis, 
founded  upon  the  fact  that  the  pendulum  vibrates 
constantly  in  one  plane. 

¥*raciical  Questions. — 1.  When  an  apple  falls  to  the  ground,  how  much 
does  the  earth  rise  to  meet  it  ?  2.  What  causes  the  sawdust  on  a  mill-pond  to 
collect  in  large  masses  ?  3.  Will  a  body  weigh  more  in  a  valley  or  on  a  moun- 
tain? 4.  Will  a  pound  weight  fall  more  slowly  than  a  two-pound  weight? 
5.  How  deep  is  a  well  if  it  takes  three  seconds  for  a  stone  to  fall  to  the  bottom 
of  it?  6.  Is  the  centre  of  gravity  always  within  a  body, — as,  for  example,  a 
ring  ?  7.  If  two  bodies,  weighing  respectively  2  and  4  Ibs.,  be  connected  by  a 
rod  2  feet  long,  where  is  the  centre  of  gravity  ?  8.  In  a  ball  of  equal  density 
throughout,  where  is  the  centre  of  gravity?  9.  Why  does  a  ball  roll  down 
hill  ?  1 0.  Why  is  it  easier  to  roll  a  round  body  than  a  square  one  ?  11.  Why 
is  it  easier  to  tip  over  a  load  of  hay  than  one  of  stone  ?  12.  Why  is  a  pyramid 
ihe  stablest  of  structures  ?  13.  When  a  hammer  is  thrown,  on  which  end  does 
it  always  strike?  14.  Why  does  a  rope-walker  carry  a  heavy  balancing-pole  ? 
15.  What  would  become  of  a  ball  if  dropped  into  a  hole  bored  through  the 
centre  of  the  earth  ?  1 G.  Would  a  clock  lose  or  gain  time  if  carried  to  the  top 
of  a  mountain  ?  If  carried  to  the  North  Pole  ?  1 7.  In  the  winter,  would  you 
raise  or  lower  the  pendulum-bob  of  your  clock?  18.  Why  is  the  pendulum- 
bob  always  made  flat?  19.  What  beats  off  the  time  in  a  watch?  20. 
What  should  be  the  length  of  a  pendulum  to  vibrate  minutes  at  the  latitude  of 
New  York?  Solution— (I  sec.)2  :  (60  sec.)2  ::  39.1  in.  :  x  =  2.2  +  miles.  21. 
What  should  be  the  length  of  the  above  to  vibrate  half-seconds  ?  Quarter- 
seconds  ?  Hours  ?  22.  Two  pendulums  are  respectively  16  and  64  inches 
in  length.  What  is  their  proportionate  time  of  vibration?  23.  Why,  wheq 
you  are  standing  erect  against  a  wall,  and  a  piece  of  money  is  placed  between 
your  feet,  can  yo»  not  stoop  forward  and  pick  it  up  ?  24.  If  a  tower  were  1C8 
feet  high,  with  what  velocity  would  a  stone,  dropped  from  the  summit,  striko 
the  ground?  25.  A  body  falls  in  5  seconds ;  with  what  velocity  does  it  strike 
the  ground  ?  26.  How  far  will  a  body  fall  in  10  seconds  ?  With  what  velocity 
will  it  strike  the  ground?  27.  A  body  is  thrown  upward  with  a  velocity  of 
192  feet  the  first  second ;  to  what  height  will  it  rise  ?  (This  problem  is  to 
be  solved  as  if  it  read,  "How  far  must  a  body  fall  to  gain  a  velocity  of  192 
feet  ?")  28.  A  ball  is  shot  upward  with  a  velocity  of  256  feet ;  to  what  height 


6  4  NA  TUBA  L  PHIL  O SOPHY. 

will  it  rise?  How  long  will  it  continue  to  ascend  ?  29.  Why  do  not  drops  of 
water,  falling  from  the  clouds,  strike  with  a  force  proportional  to  the  laws  of 
falling  bodies  ?  Ans. — Because  they  are  so  small  that  the  resistance  of  the  air 
nearly  destroys  their  velocity.  If  it  were  not  for  this  wise  provision,  a  shower 
of  rain-drops  would  be  as  fatal  as  one  of  Mini6  bullets.  30.  Are  any  two 
plumb-limes  parallel  ?  3 1 .  A  stone  let  fall  from  a  bridge  strikes  the  water 
in  3  seconds.  What  is  the  height  ?  32.  A  stone  falls  from  a  church-steeple 
in  4  seconds.  What  is  the  height  of  the  steeple?  33.  How  far  would  a  body 
fall  in  the  first  second  at  a  distance  of  12,000  miles  above  the  earth's  surface?  #_. 
34.  A  body,  at  the  surface  of  the  earth,  weighs  100  tons  ;  what  would  be  its 
weight  1,000  miles  above?  35.  A  boy,  wishing  to  find  the  height  of  a  steeple, 
lets  fly  an  arrow  that  just  reaches  the  top  and  then  falls  to  the  ground.  It  is  in 
the  air  6  seconds.  Required  the  height?  36.  A  cat  let  fall  from  a  balloon 
reaches  the  ground  in  10  seconds.  Required  the  distance  ?  37.  In  what  time 
will  a  pendulum  40  feot  long  make  a  vibration  ?  38.  Two  meteoric  bodies  in 
space  are  12  miles  apart.  They  weigh  respectively  100  and  200  Ibs.  If  they 
should  fall  together  by  force  of  their  mutual  attraction,  what  portion  of  the 
distance  would  be  passed  over  by  each  body  ?  39.  If  a  body  weighs  2,000  Ibs. 
upon  the  surface  of  the  earth,  what  would  it  weigh  2,000  miles  above  ?  500 
miles  above  ?  40.  At  what  distance  above  the  earth  will  a  body  fall,  the  first 
X  second.  2iy3  inches  ?  41.  How  far  will  a  body  fall  in  8  seconds  ?  In  the  8th 
second  ?  In  10  seconds  ?  In  the  30th  second  ? 


The  ancient  methods  ofkeeping  time  were  simple  indeed.  The  sun-dial  was 
doubtless  the  earliest  device  ;  afterward  the  clepsydra  was  employed.  This 
consisted  of  a  vessel  containing  water,  which  slowly  escaped  into  a  dish  be' 
low.  In  this  was  a  floating  body  which,  by  its  height,  indicated  the  lapse  of 
time.  King  Alfred  the  Great,  we  read,  used  candles  of  a  uniform  size,  six  of 
which  lasted  a  day.  He  surrounded  these  with  cases  of  horn  as  a  protection 
from  currents  of  air.  From  a  mere  fancied  derivation  of  this  kind,  some  have 
spelled  the  word  lantern,  lanthorn.  Clocks  were  used  in  Europe  as  early  as 
the  llth  century.  The  application  of  the  pendulum  was  made  in  the  early  part 
of  the  17th  century.  The  first  clock  made  in  England,  about  A.  r>.  1288,  was 
considered  of  so  much  importance,  that  a  high  official  was  appointed  to  take 
charge  of  it.  The  clocks  of  the  Middle  Ages  were  extremely  elaborate.  They 
indicated  the  motions  of  the  heavenly  bodies  ;  birds  came  out  and  sang  songs, 
cocks  crowed  and  trumpeters  blew  their  horns  ;  chimes  of  bells  were  sounded, 
and  processions  of  dignitaries  and  military  officers,  in  fantastic  dress, 
marched  in  front  of  the  dial  and  gravely  announced  the  time  of  day.  Watches 
were  made  at  Nuremberg  in  the  15th  century.  They  were  styled  Nuremberg 
eggs.  In  the  16th  century  they  were  in  common  use.  Many  were  as  small  as 
the  watches  of  the  present  day,  while  others  were  as  large  as  a  dessert-plate. 
They  had  no  minute  or  second  hand,  and  required  winding  twice  per  day. 
They  were  extremely  cumbersome,  containing  about  800  pieces.  In  1658,  Dr. 
Hare  invented  the  main-spring.  This  gave  to  watches  the  accuracy  of  the 
pendulum.  Waltham  watches  have  but  120  pieces  in  all.  Chronometers  are 
now  made  so  perfect  as  not  to  vary  a  minute  in  six  months. 


MOTION. 

MOTION  is  a  change  of  place.  Absolute  motion  is 
a  change  without  reference  to  any  other  object. 
Relative  motion  is  a  change  with  reference  to  some 
other  object.  Rest  also  is  either  Absolute  or  Rela- 
tive. Ex. :  We  are  in  absolute  motion  with  the 
earth  as  it  flies  through  space ;  when  we  walk,  we 
judge  of  our  motion  by  the  objects  around  us ;  a 
man  on  a  steamer  is  in  motion  with  regard  to  the 
shore,  but  at  rest  with  reference  to  the  objects  on 
the  deck  of  the  vessel.  Nothing  is  in  absolute  rest. 
Motion  seems  to  be  a  law  of  Nature.  Velocity  is  the 
rate  at  which  a  body  moves.  Force  is  that  which 
tends  to  produce  or  destroy  motion. 

RESISTANCES  TO  MOTION. — The  principal  are,  Fric- 
tion, Resistance  of  the  air,  and  Gravity.  (1)  Friction 
is  the  resistance  caused  by  the  surface  over  which 
a  body  moves.  It  is  of  two  kinds,  sliding  and  Tolling. 
If  the  surface  of  a  body  could  be  made  perfectly 
smooth,  there  would  be  no  friction ;  but  in  spite  of 
the  most  exact  polish,  the  microscope  reveals 
minute  projections  and  cavities.  We  fill  these 
with  oil  or  grease,  and  thus  diminish  friction.  Fric' 


68  NATURAL  PHILOSOPHY. 

tion,  between  different  bodies,  varies  curiously. 
Between  like  substances  it  is  greater  than  between 
unlike.  Friction  is  of  great  value  in  common  life. 
Without  it,  nails,  screws,  and  strings  would  be 
useless ;  engines  could  not  draw  the  cars ;  we 
could  hold  nothing  in  our  hands  ;  everywhere  we 
would  walk  as  on  glassy  ice.  (2)  Resistance  of  the 
air.  The  resistance  which  a  body  meets  in  passing 
through  air  or  water  is  caused  by  the  particles 
which  it  must  displace.  This  increases  according  to 
the  square  of  the  velocity.  Thus,  if  we  wish  to  double 
our  speed  in  running  we  must  displace  twice  as  much 
air,  and  in  half  the  time ;  hence,  the  force  must  be 
quadrupled.  (3)  Gravity  tends  to  draw  all  bodies 
to  rest  upon  the  earth. 

MOMENTUM  is  the  quantity  of  motion  in  a  body. 
It  is  equal  to  the  weight  of  the  body  multiplied  by 
its  velocity  per  second,  expressed  in  feet.  Ex. :  A 
stone,  weighing  5  Ibs.,  thrown  with  a  velocity  of  20 
feet  per  second,  has  a  momentum  of  100  pounds. 

The  striking  force  of  a  body  is  equal  to  its  weight 

multiplied  by  the  square  of  its  velocity.     (See  p.  81.) 

Ex. :  A  bullet  weighing  2  ounces,  fired  with  a  velocity 

of  1,400  feet  per  second,  would  strike  with  a  force  of 

245,000  Ibs.     Place  a  hammer  on  the  head  of  a  nail, 

and,  though  you  push  with   all  your  might,  yon 

cannot  stir  it.      Swing  the  hammer  by  the  handle 

and  let  it  fall  upon  the  nail,  and  the  blow  will  bury 

it  to  the  head.     On  the  other  hand,  a  large  body 


MOTION. 


69 


may  be  moving  very  slowly  and  yet  have  an  im- 
mense momentum.  An  iceberg,  with  a  scarcely 
perceptible  motion,  will  crush  the  strongest  man-of- 
war  as  if  it  were  an  egg-shell.  Those  who  have 
stood  on  a  wharf  have  noticed  with  what  prodigious 
force  large  vessels  grind  against  each  other  by  the 
slow  movement  of  the  tide.  Soldiers  have  thought 
to  stop  a  spent  cannon-ball  by  putting  a  foot  against 
it,  but  have  found  its  momentum  sufficient  to  break 
a  leg. 

Motion  is  not  imparted  instantaneously. — "We  press 
with  all  our  strength  against  a  large  stone.  At  first  it 
does  not  stir.  But  the  motion  is  transmitted  from 
the  molecules  we  touch  with  our  hands,  particle  by 
particle,  until  it  reaches  the  whole  body,  and  the 
stone  yields.  A  horse  will  pull  at  a  heavy  load  for 
some  moments  before  he  starts  it;  if  he  should 
spring  forward  suddenly,  he  would  be  likely  to  break 
his  harness.  It  is  said  that  it  would  require  a  half 
minute  for  a  force  applied  at  one  end  of  a  mile  of 
railroad-iron  to  move  the  last  rail.  A  stone  thrown 
against  a  pane  of  glass  shatters  it ;  but  a  bullet  fired 
through  it  will  only  make  a  clean,  round  hole.  The 
reason  is,  that  the  hole  is  made  and  the  bullet  gone 
before  the  motion  has  time  to  pass  into  the  sur- 
rounding particles.  A  tallow  candle  may  be  fired 
through  a  board,  because  it  pierces  it  so  quickly  that 
the  particles  have  no  time  to  yield.  Its  slight  cohe- 
sion, multiplied  by  its  velocity,  is  greater  than  the 
cohesion  of  the  board. 


yo  NATURAL  PHILOSOPHY. 

IST  LAW  OF  MOTION. — A  body  once  set  in  motion 
tends  to  move  forever  in  a  straight  line.  This  is  but 
another  statement  of  the  property  of  inertia,  of 
which  we  have  already  spoken.  There  is  a  curious 
illustration  of  it  seen  in  the  swinging  of  a  pendulum. 
A  pendulum,  made  to  vibrate  with  the  least  possible 
friction,  is  placed  under  the  receiver  of  an  air-pump. 
The  more  perfectly  the  air  is  exhausted  the  longer 
it  will  vibrate.  In  the  best  vacuum  we  can  produce, 
it  will  swing  for  twenty-four  hours.  It  is  supposed 
that  if  all  the  "  resistances  to  motion"  were  removed, 
the  pendulum  would  vibrate  forever.  Philosophers 
can  explain  this  only  on  the  supposition  that  a  body, 
once  started,  tends  to  move  forever  in  a  straight 
line.  For  reasons  which  are  obvious,  no  experiment 
can  be  performed  which  will  directly  prove  the  law. 
We  can  see  the  principle,  however,  in  combination 
with  the  second  law  of  motion. 

2r  LAW  OF  MOTION. — A  force  acting  upon  a  body  in 
motion  or  at  rest,  produces  the  same  effect,  whether  it 
acts  alone  or  with  other  forces. 

All  bodies  upon  the  earth  are  in  constant  motion, 
and  yet  we  move  anything  with  the  same  ease  that 
we  should,  were  the  earth  at  rest.  We  throw  a  stone 
directly  at  an  object  and  hit  it,  yet,  within  the 
second,  the  mark  has  gone  forward  many  feet.*  A 
ball  thrown  up  into  the  air  with  a  force  that  would 
cause  it  to  rise  50  feet,  will  ascend  to  that  height 
whatever  horizontal  wind  may  be  bio  wing  at  the  time. 
If  a  cannon-ball  or  a  bullet  be  thrown  horizontally,  it 

*  The  earth  moves  forward  in  Its  orbit  about  the  sun  at  the  rate  of  18  mile* 
per  second.  See  Fourteen  Weeks  in  Astronomy,  p.  106. 


MOTION. 


\vill  fall  just  as  fast  and  strike  the  earth  just  as 
soon  as  if  dropped  to  the  ground  from  the  muz- 
zle of  the  gun.  In  Fig.  24,  D  is  an  arm  driven 
by  a  wooden 
spring,  E,  and 
turning  on  a 
hinge  at  C. 
At  D  is  a  hol- 
low contain- 
ing a  bullet 
so  arranged 
that  when  the 
arm  is  sprung, 
it  will  throw  the  ball  in  the  line  F  K.  At  F  is  a 
similar  ball,  supported  by  a  thin  slat,  G,  and  so 
arranged  that  the  same  blow  which  throws  the 
ball  D,  will  let  the  baU  F  fall  in  the  line  F  H. 
It  will  be  found  that  the  two  balls  will  strike  the 
floor  together.  This  holds  true,  no  matter  how  far 
the  ball  D  may  be  thrown.  We  here  see  that  the 
force  of  gravity  produces  the  same  effect  whether  it 
acts  alone  or  in  combination  with  another  force. 

COMPOUND  MOTION. — Let  a  ball  at  A  be  struck  by 
a  force  which  would 
drive  it  in  the  direc- 
tion A  B,  and  also 
at  the  same  instant 
by  another  which 
would  drive  it  toward  D ;  the  ball  will  move  in  the 
direction  A  C.  The  figure  A  B  C  D  is  termed  the 


Fig.  25. 


NATURAL  PHILOSOPHY. 


"Parallelogram  of  the  Forces,"   and  the  diagonal 
A  C  the  "  Kesultant." 

COMPOSITION  OF  FORCES. — Whenever   a    body  is 
Fig.  26.  acted  upon  by 

two  forces,  we 
draw  lines  rep- 
resenting these 
directions,  and 
mark  distances 
A  D  and  A  B, 
whose  lengths 
represent  their 
comp  arative 
velocities.  We 
next  complete 
the  parallelo- 
gram and  draw  the  diagonal  A  C,  which  denotes  the 
resultant  of  these  forces,  or  the  direction  in  which  the 
body  will  move.  If  more  than  two  forces  act,  we 
find  the  resultant  of  two,  then  of  that  resultant  and 
a  third  force,  and  so  on. 

Illustrations. — We  have  many  illustrations  of  com- 
pound motion  in  common  life.  A  person  wishes  to 
row  a  boat  across  a  swift  current.  It  carries  him 
down  stream.  He  steers,  therefore,  toward  a  point 
above  that  which  he  wishes  to  reach,  and  so  goes 
directly  across. — While  riding  on  a  car,  we  throw  a 
stone  at  some  object  at  rest.  The  stone,  having  the 
motion  of  the  train,  strikes  just  as  far  ahead  of  the 
object  as  it  would  have  gone  had  it  remained  on  the 


COMPOUND  MOTION. 


73 


Fig.  27. 


N 


train.  In  order  to  hit  the  mark,  we  should  have 
aimed  a  little  back  of  it. — The  circus-rider  wishes, 
while  riding  his  horse  at  full  speed,  to  jump  through 
a  hoop  suspended  before  him.  He  simply  springs 
directly  upward.  He  goes  forward  by  the  motion 
which  he  had  when  he  leaped  from  the  horse.  The 
resultant  motion  carries  him  through  the  hoop  and 
he  alights  upon  the  saddle  on  the  other  side. — A 
person  riding  in  a  coach  drops  a  cent  to  the  floor.  It 
falls  in  a  vertical  line  and  strikes  where  it  would  were 
the  coach  at  rest. — A  bird,  beating  the  air  with  both 
its  wings,  flies  in  a  direction  between  that  of  the 
two. 

RESOLUTION  OF  FORCES. 
— This  is  the  reverse  of 
the  "  Composition  of 
Forces."  It  consists  in 
finding  what  two  forces 
are  equivalent  to  a  given 
force.  It  is  attained  by 
drawing  a  parallelogram 
having  the  given  force 

for    a    diagonal.      Ex. :  '  "H 

There  is  a  wind  blowing 
from  the  West  against 
G  H,  the  sail  of  a  vessel  fi 

going  North.  We  can  resolve  the  wind-force  B  D 
into  the  two  forces  B  E  and  B  C.  The  former, 
blowing  parallel  to  the  sail,  is  of  no  use ;  the  latter 
is  perpendicular  to  it,  and  hence  tends  to  drive  the 

4 


NATURAL  PHILOSOPHY. 


Fig.  29. 


vessel  before  it  in  a  northeast  direction.      Again, 
resolving   B    D   in   Fig.   28,  which   represents   the 
vertical  force  B  C  in  Fig.  27, 
we    find   that   it  is  equivalent 
to  two  forces,  B  E  and  B    C. 
The  former  pushes   the  vessel 
sidewise,  but  it  is  mainly  coun- 
teracted by  the   shape  of  the 
keel  and  the  action  of  the  rud- 
der.     The  latter  is  parallel  to 
the  course  of  the  ship,  and  hur- 
ries it   along  northward.      By 
shifting  the  rigging,  one  vessel 
will  sail  into  harbor  while  an- 
other is  sailing  out,  both 
driven    by    the    same 
wind.     Figs.  29  and  30 
show  how,  by  twice  re- 
solving the  force  of  the 
wind  from  the  "West,  as 
in  the  last  figures,  when 
the  sail  G  H  is  placed 
in  the  new  position,  we 
have  (Fig.  30)  a  force, 
B  C,  which  drives  the 
vessel  southward.      If 
S  a  vessel  should  *wish  to 

sail  directly  W.  against  this  wind  we  have  supposed, 
it  would  tack  alternately  NW.  and  SW.  In  this  way 
it  could  go  almost  into  the  "teeth  of  the  wind." 


COMPOUND  MOTION. 


75 


In  a  similar  manner  we  may  resolve  the  three 
forces  which  act  upon  a  kite — viz.,  the  pull  of  the 
string,  the  force  of  the  wind, 
and  its  own  weight.  In  Fig. 
27  let  G  H  represent  the  face 
of  the  kite.  "We  can  resolve 
B  D,  the  force  of  the  wind, 
into  B  C  and  B  E.  We 
next  resolve  B  D,  in  Fig. 
28,  which  corresponds  to 
B  C  in  Fig.  27,  into  B  E 
and  B  C.  We  then  have  a 
force,  B  C,  which  overcomes 
the  weight  of  the  kite  and 
also  tends  to  lift  it  upward. 
The  string  pulls  in  the  direction  D  B,  perpendicularly 
to  the  face.  The  kite  obeys  neither  one  of  these 
forces  but  both,  and  so  ascends  in  a  direction,  D  G, 
between  the  two.  It  is  really  drawn  up  an  inclined 
plane  by  the  joint  force  of  the  wind  and  the  string. 

A  canal-boat  drawn  by  horses  is  acted  upon  by  a 
force  which  tends  to  bring  it  to  the  bank.  This 
force" may  be  resolved  into  two,  one  pulling  toward 
the  tow-path,  and  the  other  directly  ahead.  The 
former  is  counteracted  by  the  shape  of  the  boat  and 
the  action  of  the  rudder ;  the  latter  draws  the  boat 
forward% 

CIRCULAR  MOTION  is  a  variety  of  compound  motion 
produced  by  two  forces  called  the  Centrifugal  and 
the  Centripetal.  The  former  (centrum,  the  centre,  and 


76 


NA  TUBAL  PHIL  OSOPHY. 


fugio,  to  flee)  tends  to  drive  a  body  from  the  centre. 
The  latter  (centrum,  the  centre,  and  peto,  to  seek) 
tends  to  draw  a  body  toward  the  centre. 

The  motion  of  the  heavenly  bodies  presents  the 
grandest  illustration  of  the  operation  of  these  forces. 
The  earth,  when  first  formed,  we  may  suppose,  was 
hurled  into  space  from  the  hand  of  the  Creator  with 
a  force  which  would  send  it  along  the  line  B  C  in 
Fig.  31.  According  to  the  law  of  inertia  it  would 
never  lose  this  force,  but  would  continue  to  move 
forever  in  a  straight  line.  Being  attracted,  how- 
ever, by  the  sun  in  the  direction  B  S,  it  passes  along 

Fig.  31. 


the  line  B  D,  which  is  the  resultant  of  these  two 
forces.  Should  the  earth  ever  lose  its  own  motion, 
it  would  fall  into  the  sun  and  feed  that  central  fire. 
Should  the  attraction  of  the  sun  cease,  it  would  fly 
off  with  headlong  speed  into  the  icy,  cheerless  regions 
of  space. 


CIRCULAR  MOTION. 


77 


Examples  abound  in  common  life.  Water  flies 
from  a  grindstone  on  account  of  the  centrifugal 
force  produced  in  the  rapid  revolution,  which  over- 
comes the  force  of  adhesion.  In  factories,  grind- 
stones are  sometimes  revolved  with  such  velocity 
that  the  centrifugal  force  overcomes  the  force  of 
cohesion,  and  the  ponderous  stones  fly  into  fragments. 
A  pail  full  of  water  may  be  whirled  around  so  rap- 
idly that  none  will  spill  out,  because  of  the  centrif- 
ugal force  which  overcomes  the  force  of  gravity. 
When  a  horse  is  running  around  a  small  circle  he 
bends  inward  to  overcome  the  centrifugal  force. 

The  rapid  revolution  of  the  earth  on  its  axis  tends 
to  throw  off  all  bodies  headlong  into  space.  As  this 
force  acts  contrary  to  that  of  gravity,  it  diminishes 
the  weight  of  all  bodies  at  the 
equator,  where  it  is  greatest, 
-sfa.  It  also  tends  to  drive  the 
water  on  the  earth  from  the 
poles  toward  the  equator,  and 
in  consequence  to  heap  it 
up  in  the  equatorial  region 
of  the  ocean.  Were  the  ve- 
locity of  the  earth's  rotation  to 
diminish,  the  water  would  run 
back  toward  the  poles,  and  tend 
to  restore  the  earth  to  a  spherical  form.  This  in- 
fluence is  well  illustrated  by  the  apparatus  shown 
in  the  figure.  The  hoop  is  made  to  slide  upon  its 
axis,  and  if  it  is  revolved  rapidly  it  will  assume  an 


•78  NATURAL  PHILOSOPHY. 

oval  form,  bulging  out  more  and  more  as  the  velocity 
is  increased.* 

THIED  LAW  OF  MOTION. — Action  is  equal  to  reaction, 
and  in  the  contrary  direction.  A  bird  in  flying  beats 
the  air  downward,  but  the  air  reacts  and  supports 
the  bird.  The  boatman  pushes  with  his  pole  against 
the  dock,  and  by  the  reaction  his  boat  is  driven  from 
the  shore.  The  oarsman  strikes  the  water  backward 
with  the  same  force  that  his  boat  moves  forward. 
The  swimmer  kicks  with  his  feet  against  the  water, 
which  reacts  and  sends  him  ahead.  The  powder  in 
a  gun  explodes  with  equal  force  in  every  direction, 
driving  the  gun  backward  and  the  ball  forward,  each 
with  the  same  momentum.  Their  relative  velocities 
vary  with  their  respective  weights  ;  thte  heavier  the 
gun  the  less  will  the  recoil  be  noticed.  When  we 
spring  from  a  boat,  unless  we  are  cautious,  the  re- 
action will  drive  it  away  from  the  shore.  When  we 
jump  from  the  ground,  we  push  the  earth  from  us, 
while  it  reacts  and  pushes  us  away  from  it ;  we  sep- 
arate from  each  other  with  equal  momenta,  and  our 
velocity  is  as  much  greater  than  that  of  the  earth 
as  we  are  lighter.  We  cannot  jump  from  a  soft  sur- 
face, because  it  yields ;  but  a  spring-board,  which 
reacts  more  promptly,  aids  us.  We  walk  by  reason 

*  This  apparatus  is  always  accompanied  by  a  variety  of  objects 
which  may  be  used  to  illustrate  very  beautifully  the  principle 
that  all  bodies  tend  to  revolve  about  their  shortest  diameters. 
This  is  an  assurance  that  the  earth  will  never  change  its  axis  of 
rotation  while  it  retains  its  present  form. 


REFLECTED  MOTION. 


79 


of  the  reaction  of  the  ground  on  which  we  tread. 
Thus  at  every  step  we  take,  we  cause  the  earth  to 
move.*  The  apparatus  shown  in  the  figure  consists 
of  ivory  balls  so  hung  as  to  readily 
vibrate.  If  a  ball  be  let  fall  from  one 
side  it  strikes  the  second  ball,  which 
reacts  with  an  equal  force  and  stops  the 
motion  of  the  first,  but  transmits  the 
motion  to  the  third ;  that  acts  in  the  same  manner, 
and  so  on  through  the  series,  each  acting  and  re- 
acting until  the  last  ball  is  reached ;  this  reacts  and 
then  bounds  off,  rising  as  high  as  the  first  ball  fell 
(except^the  loss  caused  by  friction).  If  two  balls  be 
raised,  two  will  fly  off  at  the  opposite  end ;  if  two 
be  let  fall  from  one  side  and  one  from  the  other, 
they  will  respond  alternately  from  either  side. 

EEFLECTED  MOTION. — This  is  pro- 
duced by  the  reaction  of  any  surface   A 
against   which    an   elastic   body   is   B 
thrown.     If  a  ball  be  thrown  in  the 

C1 

direction  O  B  against  the  surface  A  C, 

it  will  rebound  in  the  line  B  E.     The  angle  O  B  P 

*  No  force  in  nature  can  be  wasted.  It  must  accomplish  some- 
thing. "  A  blow  with  a  hammer  moves  the  earth.  A  boy  could 
in  time  draw  the  largest  ship  across  the  harbor  in  calm  weather." 

"  Water  falling  day  by  day 
Wears  the  hardest  rock  away." 

Statues  are  worn  smooth  by  the  constant  kissing  of  enthusiastic 
worshippers.  Stone  steps  are  hollowed  by  the  friction  of  many 
feet.  The  ocean  is  rilled  by  small  drops  which  fall  from  the 
clouds.  We  may  notice  none  of  these  forces  singly,  but  their 
effects  in  the  aggregate  startle  us. 


g0  NATURAL  PHILOSOPHY. 

(the  angle  of  incidence)  will  be  equal  to  the  angle 
PER  (the  angle  of  reflection). 

MOTION  IN  A  CURVED  LINE. — "Whenever  two  or  more 
instantaneous  forces  act  upon  a  body,  the  resultant 
is  a  straight  line.  When  one  is  instantaneous,  an_d 
the  other  continuous,  it  is  a  curved  line.  When  a 
body  is  thrown  into  the  air,  unless  it  be  in  a  ver- 
tical line,  it  is  acted  upon  by  the  instantaneous 
force  of  projection  and  the  continuous  force  of 
gravity,  and  so  passes  through  a  line  which  curves 
toward  the  earth. 

PERPETUAL  MOTION. — Nothing  can  be  more  utterly 
impracticable  than  to  make  a  machine  capable  of 
perpetual  motion.  No  machine  can  produce  power ; 
it  can  only  direct  that  which  is  applied  to  it.  In  all 
machinery  there  is  friction;  this  must  ultimately 
exhaust  the  power  and  bring  the  motion  to  rest. 
These  principles  show  the  futility  of  all  such  at- 
tempts. 

Practical  Questions. — 1.  Can  a  rifle-ball  be  fired  through  a  handkerchief 
suspended  loosely  from  one  corner  ?  2.  A  rifle-ball  thrown  against  a  board 
standing  edgewise,  will  knock  it  down ;  the  same  bullet  fired  at  the  board,  will 
pass  through  it  without  disturbing  its  position.  Why  is  this  ?  3.  Why  can 
a  boy  skate  safely  over  a  piece  of  thin  ice,  when,  if  he  should  pause,  it  would 
break  under  him  directly?  4.  Why  can  a  cannon-ball  be  fired  through  a  door 
standing  ajar,  without  moving  it  on  its  hinges?  5.  Why  can  we  drive  on 
the  head  of  a  hammer  by  simply  striking  the  end  of  the  handle  ?  6.  Suppose 
you  were  on  a  train  of  cars  moving  at  the  rate  of  30  miles  per  hour ;  with  what 
force  would  you  be  thrown  forward  if  the  train  were  stopped  instantly  ?  7.  In 
what  line  does  a  stone  fall  from  the  masthead  of  a  vessel  in  motion  ?  8.  If  a 
ball  be  dropped  from  a  high  tower  it  will  strike  the  ground  a  little  east  of  a  ver- 
tical line.  Why  is  this  ?  9.  It  is  stated  that  a  suit  was  once  brought  by  the 
driver  of  a  light  wagon  against  the  owner  of  a  coach  for  damages  caused  by  a 
collision.  The  complaint  was  that  the  latter  was  driving  so  fast  that  when  the 
two  carriages  struck,  the  driver  of  the  former  was  thrown  forward  over  the 
dashboard.  On  trial  he  was  nonsuited,  because  his  own  evidence  showed  him 


MOTION.  g  I 

to  be  the  one  who  was  driving  at  the  unusual  speed.  Explain.  1 0.  Suppose 
a  train  moving  at  the  rate  of  30  miles  per  hour :  on  the  rear  platform  is  a  cannon 
aimed  parallel  to  the  track  and  in  a  direction  precisely  opposite  to  the  motion  of 
the  car.  Let  a  ball  be  discharged  with  the  exact  speed  of  the  train ;  where 
would  it  fall  ?  11.  Suppose  a  steamer  in  rapid  motion,  and  on  its  deck  a  man 
jumping.  Can  he  j  ump  farther  by  leaping  the  way  the  boat  is  moving  or  in  the 
opposite  direction?  12.  "Why  is  a  "running  jump"  longer  than  a  standing 
one?  13.  If  a  stone  be  dropped  from  the  masthead  of  a  vessel  in  motion, 
will  it  strike  the  same  spot  on  the  deck  that  it  would  if  the  vessel  were  at 
rest?  14.  Could  a  party  play  ball  on  the  deck  of  the  Great  Eastern  when 
steaming  along  at  the  rate  of  20  miles  per  hour,  without  making  allowance  for 
the  motion  of  the  ship  ?  1.1.  Since  action  is  equal  to  reaction,  why  is  it  not 
as  dangerous  to  receive  the  "  kick"  of  a  gun  as  the  force  of  the  bullet  ?  16. 
If  you  were  to  jump  from  a  carriage  in  rapid  motion,  would  you  leap  directly 
toward  the  spot  on  which  you  wished  to  alight  ?  1 7.  If  you  wished  to  shoot  a 
bird  in  swift  flight,  would  you  aim  directly  at  it?  18.  At  what  parts  of  the 
earth  is  the  centrifugal  force  least  ?  19.  What  causes  the  mud  to  fly  from  the 
wheels  of  a  carriage  in  rapid  motion  ?  20.  What  proof  have  we  that  the 
earth  was  once  a  soft  mass?  21.  On  a  curve  in  a  railroad,  one  track  is  always 
higher  than  "the  other.  Why  is  this  ?  22.  What  is  the  principle  of  the  sling  ? 
23.  The  mouth  of  the  Mississippi  River  is  about  2£  miles  farther  from  the  cen- 
tre of  the  earth  than  its  source.  In  this  sense  it  may  be  said  to  "  run  up  hill.1' 
What  causes  this  apparent  opposition  to  the  attraction  of  gravity  ?  24.  Is  it  ^ 
action  or  reaction  that  breaks  an  egg,  when  I  strike  it  against  the  table?  25. 
Was  the  man  philosophical  who  said  that  it  "  was  not  the  falling  so  far,  but 
the  stopping  so  quick,  that  hurt  him?"  26.  If  one  person  runs  against  an- 
other, which  receives  the  greater  blow?  27.  Would  it  vary  the  effect  if  the 
two  persons  were  running  in  opposite  directions  ?  In  the  same  direction  ?  / 
2  8 .  Why  cannot  you  fire  a  rifle-ball  around  a  hill  ?  29.  Why  is  it  that  a  heavy 
rifle  '"kicks"  less  than  a  light  shot-gun?  30.  A  man  on  the  deck  of  a  largo 
vessel  draws  a  small  boat  toward  him.  How  much  does  the  ship  move  to  meet 
the  boat?  31.  Suppose  a  string,  fastened  at  one  end,  will  just  support  a 
weight  of  25  Ibs.  at  the  other.  Unfasten  it,  and  let  two  persons  pull  upon  it  in 
opposite  directions.  How  much  can  each  pull  without  breaking  it  ?  32.  Can 
a  man  standing  on  a  platform-scale  make  himself  lighter  by  lifting  up  on  him- 
self? 33.  Why  cannot  a  man  lift  himself  by  pulling  up  on  his  boot-straps? 
34.  If,  from  a  gun  placed  vertically,  a  ball  were  fired  into  perfectly  still  air, 
where  would  it  fall?  35.  With  what  momentum  would  a  steamboat  weigh- 
ing 1,000  tons,  and  moving  with  a  velocity  of  10  feet  per  second,  strike  against 

a  sunken  rock? (On  page  68,  we  found  that  a  constant  force  which  tends  to 

overcome  a  continued  resistance,  like  that  of  air  or  water,  must  be  as  the 
square  of  the  velocity.  This  is  termed  its  living  force,  or  vis  viva.  This  law 
holds  good  only  in  starting  bodies  from  a  state  of  rest  and  in  low  velocities. 
At  high  rates  of  speed  less  force  is  required.  The  comparative  striking  force 
is,  however,  always  as  the  square  of  the  velocity.) 36.  With  what  mo- 
mentum would  a  train  of  cars  weighing  100  tons,  and  running  10  miles  pet 
hour,  strike  against  an  obstacle?  37.  What  would  be  the  comparative  strik- 
ing force  of  two  hammers,  one  driven  with  a  velocity  of  20  feet  per  second* 
and  the  other  10  feet  ? 


THE  ELEMENTS  OF  MACHINERY. 

THESE  are  the  simple  machines  to  which  all  ma- 
chinery can  be  reduced.  The  watch,  with  its  complex 
system  of  wheel-work,  and  the  engine,  with  its  belts, 
cranks,  and  pistons,  are  only  various  modifications 
of  some  of  the  six  elementary  forms — viz.,  the  lever, 
the  wheel  and  axle,  the  inclined  plane,  the  screw,  the 
wedge,  and  the  pulley.  These  six  may  be  still  further 
reduced  to  the  lever  and  inclined  plane.  They  are 
termed  powers,  but  do  not  produce  force ;  they  are 
only  methods  of  applying  and  directing  ii*  They 
also  enable  us  to  use  the  forces  of  Nature,  such  as 
wind,  water,  and  steam.  The  work  done  by  the 
power  is  always  equal  to  that  done  by  the  weight. 
The  law  of  all  mechanics  is — 

The  power  multiplied  by  the,  distance  through  which 
it  moves,  is  equal  to  the  weight  multiplied  by  the  distance 
through  whicli  it  moves.  Thus  1  Ib.  of  power  moving 
through  10  feet  =  10  Ibs.  of  weight  moving  through 
one  foot. 

THE  LEVER  is  a  bar  turning  on  a  pivot.  The  force 
Used  is  termed  the  power  (P),  the  object  to  be  lifted 
the  weight  (W),  the  pivot  on  which  the  lever  turns 


86 


NATURAL  PHILOSOPHY. 


the  fulcrum  (F),  and  the  parts  of  the  lever  each  side 
of  the  fulcrum  the  arms. 

The  three  classes  of  levers. — I.  Power  at  one  end, 
weight   at  the   other,    and  fulcrum  between.      II. 


Fig.  36. 


f 

Flg'  %        J  r_  —  1. 

? 

il 
1 

\ 

f 
7                       W 

A 

r            1 
w! 

Fig.  38. 


Power  at  one  end,  fulcrum  at  the  other,  and  weight 
between.  III.  "Weight  at  one  end,  fulcrum  at  the 
other,  and  power  between. 

1st  Class  of  Lever. — We  wish  to  lift  a  stone.  We 
put  one  end  of  a  handspike 
under  the  stone,  and  resting  the 
bar  on  a  block  at  F,  we  bear 
down  at  P.  A  pump-handle  is 
a  lever  of  the  first  class.  The 
hand  is  the  P,  the  water  lifted 
the  W,  and  the  pivot  the  F.  A  pair  of  scissors  is  a 
double  lever  of  the  same  class.  The  cloth  to  be  cut 
Fi.  39.  .  is  the  W,  the  hand  the  P,  and 
the  rivet  the  F. 

•2c£    Class. — We    may    also 
raise  the  stone,  as  in  Fig.  39, 
by  resting   one   end   of    the 
lever  on  the  ground,  which  acts  as  a  fulcrum,  and 


% 

^ik.      F          W 

^S&L 


MECHANICAL  POWERS.  gy 

lifting  up  on  the  bar.  An  oar  is  a  lever  of  the 
second  class.  The  hand  is  the  P,  the  boat  the  W, 
and  the  water  the  F. 

3d  Class. — The  treadle  is  a  lever  of  the  third  class. 
The  end,  C,  resting  on  the  ground  is  the  F,  the  foot 


Fig.  40. 


is  the  P,  and  the  force  is  transmitted  by  a  rod  to  the 
"W,  the  reel  above.  In  the  fishing-rod,  one  hand  is 
the  F,  the  other  the  P,  and  the  fish  the  W. 

Law  of  Equilibrium. — The  lever  is  in  equilibrium 
when  the  arms  balance  each  other.  The  distance 
through  which  the  P  and  the  W  move  depends  upon 
the  comparative  length  of  the  arms.  Let  Pd  repre- 
sent power's  distance  from  the  F,  and  Wd  weight's 
distance ;  then  if  Pd  is  twice  Wd,  the  power  will 


88 


NA  TUBAL  PHIL  OSOPHY. 


move  twice  as  far  as  the  weight.     Substituting  these 
terms  in  the  law  of  Mechanics,  we  have 

P  x  Pd  =  W  x  Wrf,  or  P  :  W  : :  Wd  :  Pd. 

In  the  first  and  second  classes,  as  ordinarily  used, 
we  gain  power  and  lose  time ;  in  the  third  class  we 
lose  power  and  gain  time. 

The  Steelyard  is  a  lever  of  the  first  class.     The 

Fig.  41. 


power  is  at  E,  the  fulcrum  at  C,  and  the  weight  at 
D.  If  the  distance  from  the  pivot  of  the  hook  D  to 
the  pivot  of  the  hook  C  is  one  inch,  and  from  the  pivot 
of  the  hook  C  to  the  notch  where  E  hangs  is  12  inches, 
then  a  1-lb.  weight  at  E  will  balance  12  Ibs.  at  W. 
If  the  steelyard  be  reversed,  as  in  Fig.  42,  then  the 
distance  of  the  fulcrum  from  the  W  is  only  J  as 
great,  and  the  same  weight  at  E  will  balance  48  Ibs. 


MECHANICAL  POWERS. 


89 


at  D.     Two  sets  of  notches  on  opposite  sides  of  the 
bar  correspond  to  these  two  positions. 

Fig.  43. 


The  Arm  is  a  lever  of  the  third  class.  The  muscle 
(Physiology,  p.  48)  is  attached  to  the  bone  of  the 
forearm,  at  a  distance  of  about  two  inches  from  the 
elbow  joint,  while  from  the  centre  of  the  palm  of  the 
hand  to  the  same  point  is  about  13  inches.  Hence 
Wd  =  13  inches  and  Pd  =  2  inches.  Therefore  the 
force  exerted  by  the  muscle  must  be  over  six  times 
the  weight  to  be  lifted  by  the  hand.  What  we  thus 
lose  in  power,  we  gain  in  the  speed  of  the  motion. 
We  desire  to  perform  quick  movements  with  our 
hands,  and  so  they  are  wisely  and  expressly  con- 
trived to  meet  our  wants. 

Bent  Lever. — In  the  hammer,  when  used  to  draw 


9° 


NATURAL  PHILOSOPHY 


a  nail,  we  have  a  good  illustration  of  a  bent  lever. 
Fig.  43.  The  real  length  of  the  arms  is  that  of 
the  straight  lines  which  correspond  to 
the  direction  in  which  the  power  and 
weight  act  with  reference  to  the  fulcrum. 
The  Compound  Lever  consists  of  sev- 
eral levers  so  connected  that  the  short 
arm  of  the  first  acts  on  the  long  arm  of  the  second, 
and  so  on  to  the  last.  If  the  distance  of  A  from  the 
F  be  four  times  that  of  B,  then  a  power  of  5  Ibs.  at 
A  will  lift  a  W  of  20  Ibs.  at  B.  If  the  arms  of  the 
second  lever  are  of  the  same  comparative  length, 
then  a  power  of  20  Ibs.  at  C  will  lift  80  Ibs.  at  E. 
In  the  third  lever,  a  power  of  80  Ibs.  at  D  will,  in 
the  same  proportion,  lift  320  Ibs.  at  G.  Thus,  with 
Fiff.  44.  this  system  of  three 

•i.....; A     levers,  a  power  of  5 

I  Ibs.  will  balance  a 
—  weight  of  320  Ibs.  In 
order,  however,  to  raise  the  weight  one  foot,  the 
power  must  pass  through  64  feet.  Hay-scales  are 
constructed  upon  the  principle  of 
the  compound  lever. 

THE  WHEEL  AND  AXLE  is  a  modi- 
fication of  the  lever.  The  wind- 
lass used  for  drawing  water  from 
a  well,  is  a  common  form.  The 
power  is  applied  at  the  handle,  the 
bucket  is  the  W,  and  the  F  is  the 
axis  of  the  windlass.  The  long  arm  of  the  lever  is 


Fig.  45. 


MECHANICAL  POWERS. 


91 


the  length  of  the  handle,  and  the  short  arm  is  the 
semi-diameter  of  the  axle.  This  is  seen  very  clearly 
in  the  cross-section  shown  Fig.  46. 

in  Fig.  45,  where  O  is  the 
F,  O  A  the  long  arm,  and 
O  B  the  short  arm.  In 
Fig.  46,  instead  of  turning 
a  handle  we  take  hold  of 
pins  inserted  in  the  rim  of 
a  wheel.  Fig.  47  represents 
a  capstan  used  on  board 
of  vessels  for  weighing  the 
anchor.  The  power  is  applied  by  means  of  hand- 
spikes which  radiate  outward  from  the  axle.  Fig.  48 
shows  a  form  of  the  capstan 
often  used  in  moving  build- 
ings, in  which  a  horse  fur- 
nishes the  power.  The  wheel 
and  axle  has  the  advan- 
tage that  it  is  a  kind  of  per- 
petual lever.  We  are  not  obliged  to  prop  up  the  W  and 
readjust  the  lever,  but  both  arms  work  continuously. 
Law  of  Equilibrium. — By  turning  the  handle  or 
wheel  around  once,  the  rope  will  be  wound  once 
around  the  axle  and  the  W  be  lifted  that  distance. 
Applying  the  law  of  Mechanics,  we  see  that  the 
Power  x  the  circumference  of  the  Wheel  =  the 
Weight  X  circumference  of  the  axle ;  or,  as  circles 
are  proportional  to  their  radii, 

P  :  W  :  :  Radius  of  the  Axle  :  Radius  of  the  Wheel. 


NATURAL  PHILOSOPHY. 

If  the  radius  of  an  axle  be  6  inches,  and  the  radius 

Fig.  48. 


of  the  wheel  24  inches,  then  the  weight  will  exceed 
the  power  four  times. 

Wheelwork  consists  of  a  series 
of  wheels  and  axles  which  act 
upon  each  other  on  the  princi- 
ple of  a  compound  lever.  The 
cogs  on  the  circumference  of  the 
wheel  are  termed  teeth,  on  the 
axle  leaves,  and  the  axle  itself  a 
pinion.  If  the  radius  of  the  wheel  F  is  12  inches, 
and  that  of  the  pinion  2  inches,  then  a  power  of  1  Ib. 
will  apply  a  force  of  6  Ibs.  to  the  second  wheel  E. 
If  the  radius  of  this  is  also  12  inches,  then  the 
second  wheel  will  apply  a  force  of  36  Ibs.  to  the 
third  wheel.  This,  acting  on  its  axle,  will  balance  a 
W  of  216  Ibs.  In  order,  however,  to  lift  this  amount, 
according  to  the  principle  already  named,  the  weight 
will  only  pass  through  -^{^  of  the  distance  of  the 


MECHANICAL  POWERS. 


93 


power.  We  thus  gain  power  and  lose  speed.  If  we 
wish  to  reverse  this  we  can  apply  the  power  to  the 
axle,  and,  with  ,  a  correspondingly  heavy  power, 
gain  speed.  This  is  the  plan  adopted  in  factories, 
where  a  heavy  water-wheel  furnishes  abundance  of 
power,  and  spindles  or  other  swift  machinery  are  to 
be  turned  very  rapidly. 

THE  INCLINED  PLANE. — If  we  wish  to  lift  a  heavy 
cask  into  a  wagon,  we  rest  one  end  of  a  plank  on 
the  wagon-box  and  the  other  on  the  ground.  We 
can  then  roll  the  cask  up  the  inclined  plane  thus 

Fig.  50. 


formed,  when  we  could  not  have  lifted  it  directly. 
When  roads  are  to  be  made  over  steep  hills,  they 
are  sometimes  constructed  around  the  hill,  like  the 
thread  of  a  screw,  or  in  a  winding  manner,  as  shown 


94  NATURAL  PHILOSOPHY. 

in  Fig.  50.  The  road  from  Callao  to  Lima,  in 
South  America,  is  said  to  be  one  of  the  longest  and 
best-made  inclined  planes  in  the  world.  It  is  six 
miles  in  length,  and  the  total  rise  is  511  feet 
Stairs  are  inclined  planes  with  steps  cut  in  them  to 
facilitate  their  ascent. 

Fig.  51.  Law  of  Equilibrium.— 

In  Fig.  51  we  see  that 
the  power  must  descend 
a  distance  equal  to  A  C 
in  order  to  elevate  the 
weight  to  the  height  C  B.  Applying  the  law  of 
Mechanics,  we  have  P  X  length  of  the  inclined 
plane  =  W  X  height  of  inclined  plane ;  hence, 

P  :  W  :  :  height  of  inclined  plane  :  length  of  inclined  plane. 

Thus,  if  we  roll  a  barrel  of  pork,  weighing  200  Ibs., 
up  a  plane  12  feet  long  and  3  feet  high  into  a  wagon, 
we  have  x  =  50  Ibs.  :  200  Ibs.  : :  3  feet  :  12  feet.  In 
this  case  we  lift  only  50  Ibs.,  or  £  of  the  barrel,  but 
we  lift  it  through  four  times  the  space  necessary  if 
we  could  have  raised  it  directly  into  the  wagon. 
We  thus  lose  speed  and  gain  power.  The  longer 
the  inclined  plane,  the  greater  the  load  we  can  lift, 
but  the  longer  it  will  take  to  do  it.  If  a  road  as- 
cends one  foot  in  100  feet,  then  a  horse  drawing  up 
a  wagon  has  to  lift  only  T^T  of  the  load,  besides 
overcoming  the  friction.  A  body  rolling  down  an 
inclined  plane  acquires  the  same  velocity  that  it 
would  in  falling  the  same  height  perpendicularly. 


MECHANICAL  POWERS.  0^ 

A  train  descending  a  grade  of  one  foot  to  100 
feet  tends  to  go  down  with  a  force  equal  to  -^  of 
its  weight.  Near  Lake  Lucerne,  Switzerland,  is  a 
valuable  forest  of  firs  on  the  top  of  an  almost  inac- 
cessible Alpine  mountain.  By  means  of  a  wooden 
trough,  the  trees  are  conducted  into  the  water 
below,  a  distance  of  eight  miles,  in  as  many  minutes. 
One  standing  near  hears  a  "roar  as  of  distant  thunder, 
and  the  next  instant  the  descending  tree  darts  past 
him  and  plunges  downward  out  of  sight.  The  force 
with  which  it  falls  is  so  prodigious,  that  if  it  jumps 
out  of  the  trough  it  is  dashed  to  pieces. 

THE  SCEEW  consists  of  an  inclined  plane  wound 
around  a  cylinder.  The  inclined  plane  forms  the 
thread,  and  the  cylinder  the  Fig.  53. 

body.  It  works  in  a  nut  which 
is  fitted  with  reverse  threads 
to  move  on  the  thread  of  the 
screw.  The  nut  may  turn  on 
the  screw,  or  the  screw  in  the 
nut.  The  power  may  be  ap- 
plied to  either  as  desired,  by 
means  of  a  wrench  or  a  lever.  The  screw  is  used  in 
presses  for  squeezing  oil  and  juices  from  apples, 
grapes,  rapeseed,  linseed,  sugar-cane,  etc. ;  for  copy- 
ing letters,  for  coining  money ;  in  vises  and  in  rais- 
ing buildings. 

Law  of  Equilibrium. — When  the  power  is  applied 
at  the  end  of  a  lever,  it  describes  a  circle  of  which 
the  lever  is  the  radius.  The  distance  through  which 


9 6  NATURAL  PHILOSOPHY. 

the  power  passes,  is  the  circumference  of  this  circle ; 
and  the  height  to  which  the  weight  is  elevated  at 
each  revolution  of  the  screw,  is  the  distance  between 
two  of  the  threads.  Applying  now  the  law  of 
Mechanics,  we  have  P  X  circumference  of  circle  = 
W  X  interval  between  the  threads ;  hence, 

P  :  W  : :  interval  :  circumference. 

The  power  of  the  screw  may  be  increased  by  length- 
ening the  lever,  or  by  diminishing  the  distance  be- 
tween the  threads. 

THE  WEDGE  usually  consists  of  two  inclined  planes 
p.    M  placed  back  to  back.     It  is  used  for 

splitting  logs  of  wood  and  blocks  of 
stone ;  for  lifting  vessels  in  the  dock ; 
and,  in  oil-presses,  for  squeezing. 
Chimneys  which  have  leaned  over, 
have  been  righted  by  wedges  driven 
in  on  the  lower  side.  Nails,  needles, 
axes,  etc.,  are  constructed  on  the  principle  of  the 
wedge. 

The  Laiv  of  Equilibrium  is,  in  theory,  the  same  as 
that  of  the  inclined  plane — viz., 

P  :  W  : :  thickness  of  wedge  :  length  of  wedge. 

In  practice,  however,  this  by  no  means  accounts  for 
its  prodigious  power.  Friction,  in  the  other  mechani- 
cal powers,  materially  diminishes  their  efficiency ;  in 
this  it  is  essential,  since,  without  it,  after  each  blow 
the  wedge  would  fly  back  and  the  whole  effect  be 
lost.  Again  :  in  the  others,  the  power  is  applied  as 


MECHANICAL  POWERS. 


97 


Fig.  55. 


a  steady  force :  in  this  it  is  a  sudden  blow,  and  is 
equal  to  the  momentum  of  the  hammer. 

THE  PULLEY  is  simply  another  form  of  the  lever 
which  turns  about  a  fixed  axis  or  fulcrum.  It  con- 
sists of  a  wheel,  within  the  grooved  edge  of  which 
runs  a  cord. 

When  we  wish  to  transmit  force  from  one  point  to 
another,  we  may  do  so  either  by  pushing  with  a  rigid 
bar,  or  by  pulling  with  a  flexible  cord.  The  advan- 
tage of  the  latter  method  is,  that  we  may 
at  the  same  time  change  the  direction 
of  the  force.  This  is  accomplished  by  a 
single^xed  pulley  y  as  in  Fig.  55.  Here 
there  can  be  no  gain  of  power  or  of  speed, 
as  the  hand  F  must  pull  down  as  much 
as  the  weight  "W,  and  both  move  with 
the  same  velocity.  It  is  simply  a  lever 
of  the  first  class  with  equal  arms.  But  its  use  is 
seen  when  we  remember  how,  by  means  of  it,  a  man 
standing  on  the  ground  Fig  m 

hoists  a  flag  to  the  top  of 
a  lofty  pole,  and  thus 
avoids  the  trouble  and 
danger  of  climbing  up  with 
it.  Two  fixed  pulleys, 
arranged  in  the  manner 
shown  in  Fig.  56,  enable 
us  to  elevate  a  heavy  load 
to  the  upper  story  of  a 
building  by  horse-power. 

5 


NATURAL  PHILOSOPHY. 


Movable  Pulley. — A  form  of  the  single  pulley,  where 
it  moves  with  the  W,  is  represented  in  Fig.  57.     In 
Fig.  57.      this,  one-half  of  the  barrel  is  sustained  by 
the  hook  F,  while  the  hand  lifts  the  other. 
Since,  then,  the  power  is  only  one-half  the 
weight,  it  must  move   through  twice  the 
space ;  in  other  words,  by  taking  twice  the 
time,  we    can   lift   twice    as   much.     Here 
power  is  gained  and  time  lost. 

We  may  also  explain  the  action  of  the 
single  movable  pulley  by  Fig.  58,  in  which 
A  represents  the  F,  E  the  W  acting  in 
the  line  O  E,  and  B  the  P  acting  in  the 
|B  line  B  P.  This  is  a  lever  of  the  second 
class ;  and  as  B  is  twice  as  far  from  A 
as  O  is,  the  power  is  only  one-half 
the  weight. 

Combinations  of  Pulleys. — 1.  In  Fig.  59,  we  have 
Fig.  60.  the  W  sustained  by  three  cords, 
each  of  which  is  stretched  by 
a  tension  equal  to  the  P,  hence 
1  Ib.  of  power  will  balance  3  Ibs. 
of  weight.  2.  In  Fig.  60,  the 
power  will  in  the  same  manner 
sustain  a  W  of  4  Ibs.,  and  must 
descend  4  inches  to  raise  the 
"W  one  inch.  3.  In  the  cord 
marked  1,  1  (Fig.  61),  each 
part  has  a  tension  equal  to  P ; 
and  in  the  cord  marked  2,  2, 


Fig.  59. 


MECHANICAL  POWERS. 


Fig.  61. 


Fig.  62. 


each  part  has  a  tension  equal  to  2P,  and  so  on  with 
the  other  cords.  The  sum  of  the  tensions  acting  on 
W  is  16,  hence  W  =  16  P. 

Fig.  62  represents  the 
ordinary  "  tackle-block" 
used  by  mechanics. 

Law  of  Equilibrium.  — 
In  all  combinations  of 
pulleys,  nearly  one-half 
the  effective  force  is  lost 
by  friction.*  In  most  of 
the  forms  in  use,  the  W 
is  equal  to  the  P  multi- 
plied by  twice  the  num- 
ber of  movable  pulleys. 

practical  Questions.—  \.  Describe  the  rudder  of  a  boat  as  a  lever  ;  a  door; 
a  door  latch;  a  lemon-squeezer;  a  pitchfork;  a  spade:  a  shovel;  a  sheep- 
shears  ;  a  poker  ;  a  pair  of  tongs  ;  a  balance  ;  a  pair  of  pincers  ;  a  wheelbarrow  ; 
a  man  pushing  open  a  gate  with  his  hand  near  the  hinge  ;  a  chopping-knife 
(Fig.  G3)  ;  a  sledge-hammer,  when  the  arm  is  swung 
from  the  shoulder;  a  nut-cracker.  2.  Show  the 
change  that  occurs  from  the  second  to  the  third 
class  of  lever,  when  you  take  hold  of  a  ladder  at  one 
end  and  raise  it  against  a  building.  3.  Why  is  a 
pinch  from  the  tongs  near  the  hinge  more  severe 
than  one  near  the  end  ?  4.  Two  persons  are  carry- 

ing a  weight  of  250  Ibs.,  hanging  between  them  from  a  pole  10  feet  in 
length.  Where  should  it  be  suspended  so  that  one  will  lift  only  50  Ibs.  ? 
5.  In  a  lever  of  the  first  class,  6  feet  long,  where  should  the  F  be  placed 
so  that  a  power  of  1  Ib.  will  balance  a  W  of  23  Ibs.  ?  6.  What  power  would 
lie  required  to  lift  a  barrel  of  pork  with  a  windlass  whose  axle  is  one 
loot  in  diameter,  and  handle  3  feet  long  ?  7.  What  sized  axle,  with  a  wheel 
6  feet  in  diameter,  would  be  required  to  balance  a  weight  of  one  ton  by  a 

*  The  work  lost  is  not  destroyed,  for  this  is  an  impossibility. 
No  force  nor  matter  has  been  destroyed,  so  far  as  we  know, 
since  the  creation  of  the  world.  The  force  is  converted  into  other 
forms  —  heat,  electricity,  etc.,  according  to  the  principle  of  the 
correlation  of  forces,  p.  316. 


1  oo  NA  TURA  L  PHIL  OSOPH  Y. 

power  of  100  Ibs. 9  8.  What  number  of  movable  pulleys  would  be  required 
to  lift  a  W  of  200  Ibs.  by  means  of  a  power  of  25  Ibs.  ?  9.  How  many  Ibs.  could 
be  lifted  with  a  system  of  4  movable  pulleys  and  one  fixed  pulley  to  change 
the  direction  of  the  force,  by  a  power  of  100  Ibs.  ?  1 0.  What  weight  could  be 
lifted  with  a  single  horse-power*  acting  on  the  system  of  pulleys  shown  in 
Fig.  62  (tackle-block)  ?  11.  What  distance  should  there  be  between  the 
threads  of  a  screw  in  order  that  a  P  of  25  Ibs.  acting  on  a  handle  three  feet  long, 
may  lift  a  ton  weight  ?  12.  How  high  would  a  P  of  12  Ibs.,  moving  16  feet 
along  an  inclined  plane,  lift  a  W  of  96  Ibs.  ?  13.  I  wish  to  roll  a  barrel  of 
flour  into  a  wagon,  the  box  of  which  is  four  feet  from  the  ground.  I  can  lift  but 
241bs.  How  long  a  plank  must  I  get  ?  14.  The  "evener"  of  a  pairof  whiffle- 
trees  is  3  feet  6  inches  in  length ;  how  much  must  the  whiffletree  be  moved  to 
give  one  horse  an  advantage  of  one-third  over  the  other  ?  15.  In  a  set  of 
three-horse  whiffletrees,  having  an  "  evener"  5  feet  in  length,  at  what  point 
should  the  plough-clevis  be  attached  that  the  single  horse  may  draw  the  same 
as  each  of  the  span  of  horses?  At  what  point  to  give  him  one-quarter  advati- 
tage  ?  16.  What  weight  can  be  lifted  with  a  power  of  100  Ibs.  acting  on  a 
screw  having  threads  one-quarter  of  an  inch  apart,and  a  lever  handle  4  feet  long  ? 
17.  What  is  the  object  of  the  big  balls  always  cast  on  the  ends  of  the  handle 
of  the  screw  used  in  presses  for  copying  letters  ?  1 8.  In  a  pair  of  steelyards 

2  feet  long,  the  distance  from  the  weight-hook  to  the  fulcrum-hook  is  2  inches  ; 
how  heavy  a  body  can  be  weighed  with  a  1-lb.  weight  at  the  further  end  ? 
19.  Describe  the  change  from  the  first  to  the  third  class  of  levers,  in  the  dif- 
ferent ways  of  using  a  pitchfork  or  spade.    2O.  Why  are  not  blacksmiths'  tongs 
and  fire-tongs  constructed  on  the  same  principle  ?    2 1 .  In  a  lever  of  the  third 
class,  what  W  will  a  P  of  50  Ibs.  balance,  if  one  arm  is  12  feet  and  the  other 

3  feet  long  ?    22.  In  a  lever  of  the  second  class,  what  W  will  a  P  of  50  Ibs. 
balance,  with  a  lever  12  feet  long,  and  W  3  feet  from  the  F  ?     23.  In  a  lever 
of  the  first  class,  what  W  will  a  P  of  50  Ibs.  balance,  with  a  lever  12  feet  long, 
and  the  F  3  feet  from  the  W?     24.  In  a  wheel  and  axle,  the  P  =  40  Ibs.,  the 
W  =  360  Ibs.,  and  the  diameter  of  the  axle  =  8  in.    Required  the  circumference 
of  the  wheel.    25.  Suppose,in  a  wheel  and  axle,  the  P  =  20  Ibs.,  the  W  =  240 
Ibs.,  and  the  diameter  of  the  wheel  =  4  feet.  Required  the  circumference  of  the 
axle?    26.  Required,  in  awheel  and  axle,  the  diameter  of  the  wheel,  the 
diameter  of  the  axle  being  10  inches,  the  P  100  Ibs.,  and  the  W  1  ton  ?     27. 
What  P  would  be  necessary  to  sustain  a  W  of  3,780  Ibs.  with  a  system  of  si$ 
movable  pulleys,  and  a  single  rope  passing  over  them  all  ?    28.  How  many 
movable  pulleys  would  be  required  to  sustain  a  W  of  420  Ibs.,  with  a  P  ol 
210  Ibs.  ? 

*A  horse-power  is  reckoned  in  Mechanics  as  a  force  which  will  lift  22. 000 
Ibs.  one  foot  in  one  minute,  without  any  assistance  of  machinery. 


of  I  iqmda  and 

'" 


The  waves  that  moan  along  the  shore, 
The  winds  that  sigh  in  blowing, 

Are  sent  to  teach  a  mystic  lorei 
Which  men  are  wise  in  knowing." 


HYDROSTATICS. 

HYDROSTATICS  treats  of  liquids  at  rest.  Its  prin- 
ciples apply  to  all  liquids ;  but  water,  on  account  of 
its  abundance,  is  taken  as  the  type  of  the  class,  and 
all  experiments  are  based  upon  it. 

I.    LIQUIDS     TRANSMIT1     PRESSURE     EQUALLY    IN    ALL 

DIRECTIONS. — This  is  the  first 
and  most  important  law.  As 
the  particles  of  a  liquid  move 
freely  among  themselves, 
there  is  no  loss  by  friction, 
and  any  force  will  be  trans- 
mitted equally,  upward, 
downward,  and  sidewise. 
Thus  if  a  bottle  be  filled 
with  water  and  a  pressure  of 
1  Ib.  be  applied  upon  the 
cork,  it  will  be  communicated 
from  particle  to  particle 
throughout  the  water.  If 
the  area  of  the  cork  be  one 
square  inch,  the  pressure  upon  any  square  inch  of 


104 


NATURAL  PHILOSOPHY. 


Fig.  66. 


the  glass  at  n,  a,  b  or  c,  will  be  equal  to  1  Ib.     If 
the  inside  surface  of  the  bottle  be  100  square  inches, 
then  a  pressure  of  1  Ib.  upon  the  cork  will  produce 
a  total  force  of  100  Ibs.,  tending  to  burst  the  bottle. 
Illustrations    of    the    transmission   of  pressure    by 
Fig.  65.  liquids.  —  Under 

some  circumstan- 
ces this  is  more 
perfect  than  that  by  solids.  Let  a  straight  tube,  A  B, 
be  filled  with  a  cylinder  of  lead,  and  a  piston,  be 
fitted  to  the  end  of  the  tube.  If  now  a  force  be  ap- 
plied at  O  it  will  be 
transmitted  with- 
out loss  to  P.  If, 
instead,  we  use  a 
bent  tube,  the  force 
will  be  transmitted 
in  the  line  of  the  arrow,  and  will  act  upon  P  but 

slightly,  if  at  all. 
If,  however,  we 
fill  the  tube  with 
water,  the  force 
will  pass  with- 
out diminution. 
With  cords,  pul- 
leys, levers,  etc., 
we  always  lose 
about  one-half 
of  the  force  by 
friction ;  but  this  "  liquid  rope"  transmits  it  with 


HYDROSTATICS. 


I05 


Fig.  68. 


no  sensible  loss.  Take  a  glass  bulb  and  stem,  as 
shown  in  Fig.  67,  and  fill  it  with  water  by  the 
process  explained  under  Thermometers.  When 
full,  if  you  are  careful  to  let  the  stem  slip  loosely 
through  the  fingers  as  the  bulb  strikes,  you  may 
pound  with  it  upon  a  smooth  surface  with  all  your 
strength.  In  this  case,  the  force  of  the  blow  is 
instantly  transmitted  from  the  thin  glass  to  the 
water,  and  that  being  almost  incompressible,  makes 
the  bulb  nearly  as  solid  as  a  ball  of  iron. 

If  a  Eupert's  drop  be  held  in  a  vial  of  water,  as  in 
Fig.  68,  and  the 
tapering  end  be 
broken,  the  force 
of  the  concussion 
will  be  transmit- 
ted to  all  parts  of 
the  glass  and  the 
vial  will  be  in- 
stantly shattered. 

Water  as  a  mechan- 
ical power. — Take  two 
cylinders,  P  and  p, 
connected  as  in  Fig. 
69,  fitted  with  pistons 
and  filled  with  water. 
Let  the  area  of  p  be 
2  inches  and  that  of  P  be  100  inches.  Then, 
according  to  the  principle  of  the  equal  pressure  of 
liquids,  a  downward  pressure  of  1  Ib.  on  each  square 


Fi"  69. 


io6 


NATURAL  PHILOSOPHY. 


inch  of  the  small  piston  will  produce  an  upward 
pressure  of  1  Ib.  on  each  square  inch  of  the  large 
piston.  Hence  a  power  of  2  Ibs.  would  lift  a  weight 
of  100  Ibs.  This  proportion  may  be  increased  by 
diminishing  the  size  of  p  and  increasing  that  of  P, 

Fig.  70. 


so  that  the  weight  of  a  girl's  hand  could  lift  a  man- 
of-war.  Water  has  been  well  termed  the  <l  seventh 
mechanical  power." 

Hydrostatic    Press. — Fig.    70   represents   a   press 
constructed  on  the  principle  just  explained.     As  the 


HYDROSTATICS.  IOy 

piston  a  is  forced  down  upon  the  water  in  the 
cylinder  A  by  the  workman,  the  pressure  is  trans- 
mitted through  the  bent  tube  of  water  d  around 
under  the  large  piston  C  which  lifts  up  the  platform 
K,  and  thus  compresses  the  bales  placed  upon  it. 
If  the  area  of  a  is  1  inch  and  that  of  C  100  inches, 
then  a  force  of  100  Ibs.  will  lift  10,000  Ibs.  Still 
further  to  increase  the  efficiency  of  this  press,  the 
handle  is  a  lever  of  the  second  class.  If  the  distance 
of  the  hand  from  the  pivot  is  ten  times  that  of  the 
piston,  a  P  of  100  Ibs.  will  produce  a  force  of  1,000 
Ibs.  at  a.  This  will  become  100,000  Ibs.  at  C.  Hence, 
with\  a  press  of  this  size,  a  power  of  100  Ibs.  will  lift 
a  weight  or  produce  a  pressure  of  100,000  Ibs.  Ap- 
plying the  principle  of  Mechanics,  we  see  that  here 
as  elsewhere  there  is  no  force  created,  but  that  P  X 
Pd  =  W  X  Wd.  The  platform  will  ascend  only 
tV^Toir  Par^  °f  the  distance  the  hand  descends.  This 
machine  is  used  for  baling  hay  and  cotton  for  trans- 
portation ;  for  launching  vessels ;  for  testing  the 
strength  of  ropes,  chains,  etc.  The  presses  employed 
for  raising  the  immense  tubes  of  the  Britannia 
Bridge  were  each  capable  of  lifting  2,672  tons,  or 
of  throwing  water  in  a  vacuum  to  a  height  of  nearly 
six  miles. 

II.   LIQUIDS  INFLUENCED  BY  GRAVITY  ALONE. — In  this 

case  there  is  no  external  pressure  applied.  The 
lower  parts  of  a  vessel  of  water  must  bear  the 
weight  of  the  upper  parts.  Thus  each  particle  of 
water  at  rest  is  pressed  downward  by  the  weight 


io8 


NATURAL  PHILOSOPHY 


of  the  minute  column  it  sustains.  It  must,  in  turn, 
press  in  every  direction  with  the  same  force,  else  it 
would  be  driven  out  of  its  place  and  the  liquid  would 
no  longer  be  at  rest.  Indeed,  when  a  liquid  is  dis- 
turbed in  any  manner  it  comes  to  rest;  i.  e.,  there  is 
an  equilibrium  established  only  when  there  is  this 
equality  of  pressure  produced.  In  consequence  of 
this  constant  pressure  the  following  laws  obtain : 

1st.  Liquids  at  rest  press  doivmvard,  upivard,  and 
sideioise  with  the  same  force. — This  may  be  illustrated 
by  the  following  experiment.  If  the  series  of  glass 


tubes  shown  in  Fig.  71  be  placed  in  a  pail  of  water, 
the  liquid  will  be  forced  up  1  by  the  upward  pressure 
of  the  water,  2  by  the  downward  pressure,  3  by  the 
lateral  pressure,  and  4  by  the  three  combined  in  dif- 
ferent portions  of  the  tube.  The  water  will  rise  in 
them  all  to  the  same  height — i.  e.,  to  the  level  of  the 
water  in  the  pail. 

2d.  The  pressure  increases  ivith  the  depth. — The 
pressure  at  the  depth  of  one  foot  is  the  weight  of 
one  cubic  foot  of  water — viz.,  62 £  Ibs.  (1,000  oz.) ;  at 


HYDE  OSTA  TICS. 


2  feet,  twice  that  amount  ;  and  so  on.*  In  sea-water 
it  is  greater,  as  that  weighs  64.37  Ibs.  per  cubic  foot. 
At  great  depths  this  pressure  becomes  enormous.  If 
a  strong  square  glass  bottle,  empty  and  firmly  corked, 
be  sunk  into  the  water,  it  will  generally  be  crushed 
inward  before  it  sinks  ten  fathoms.  It  is  said  that 
the  Greenland  whale  sometimes  descends  to  the 
depth  of  a  mile,  but  always  comes  up  exhausted  and 
blowing  blood.  When  a  ship  founders  at  sea,  the 
great  pressure  forces  the  water  into  the  pores  of  the 
wood,  so  that  no  part  can  ever  rise  again  to  the 
surface  to  reveal  the  fate  of  the  lost  vessel. 

3d'.  The  pressure  does  not  depend  on  the  shape  or  size 
of  the  vessel.  —  In  the  apparatus  shown  in  Fig.  72  the 
water  rises  to  the 
same  height  in  the 
variously  shaped 
tubes,  which  com- 
municate with 
each  other,  what- 
ever may  be  their 
form  or  size.  If 
more  water  be 
poured  in  one,  it 
will  rise  higher  in 
all  the  others. 


Fig.  72. 


*  Depth. 

Lbs.  per  sq.  foot. 

Depth. 

Lbs.  per  sq.  foot. 

1  ft. 

62.5 

100  ft. 

6,250 

10ft. 

625. 

1  mile, 

830,000 

16ft. 

1,000. 

5  miles, 

1,650,000 

no 


NATURAL  PHILOSOPHY. 


The  Hydrostatic  Bellows  consists  of  two  boards, 
each  hinged  on  one  side  and  resting  on  a  rubber 

bag,  to  which  is 
attached  an  up- 
right  tube,  A. — 
Water  is  poured 
in  at  A  until  the 
bag  and  tube  are 
filled.  The  pres- 
sure of  the  column 
of  water  in  the 
tube  lifts  the 
weights  hung  by 
crossbars  beneath. 
"Whether  we  use 
the  tube  A  or  B  will  make  no  difference  in  the  weight 
supported,  although  the  former  holds  ten  times  as 
much  water  as  the  latter.  The  tube  C,  however, 
being  much  longer,  will  exert  a  greater  pressure. 
Another  form  of  the  same  apparatus 
(Fig.  74)  consists  of  two  boards  con- 
nected by  a  bancTof  leather,  in  which 
a  tall  tube  A  is  inserted.  If  this 
be  filled  with  water,  the  pressure 
will  be  sufficient  to  lift  a  weight 
as  much  greater  than  the  weight  of 
the  water  in  the  tube  as  the  area  of 
the  bellows-board  is  greater  than  the 
area  of  the  tube.  Applying  again  the 
principle  of  Mechanics,  we  see  that  if  one  ounce  of 


HYDROSTATICS. 


Ill 


water  should  raise  a  weight  of  50  oz.  one  inch,  then 
the  water  must  fall  50  inches.  rig.  75. 

A  strong  cask  fitted  with  a 
small  pipe  30  or  40  feet  long, 
if  filled  with  water  will  burst 
asunder.  The  pressure  is  as 
great  as  if  the  tube  were  of  the 
same  diameter  as  the  cask. 
In  a  coffee  or  tea  pot  the  small 
quantity  of  liquid  in  the  spout 
balances  the  large  quantity 
in  the  vessel.  If  it  were  not 
so,N  it  would  rise  in  the  spout 
and  run  out. 

The  principle  that  a  small 
quantity  of  water  will  thus 
balance  another  quantity, 
however  large,  or  will  lift  any 
weight,  however  great,  is  fre- 
quently termed  the  "  Hydrostatic  Paradox."  We  see, 
however,  that  it  is  only  an  instance  of  the  general  law. 

4th.  Water  seeks  its  level.—  This  tendency  is  seen  es- 
pecially in  fountains  and  in  the  supply  of  water 
furnished  to  cities  from  an  elevated  reservoir.  In 
Fig.  76  the  tank  is  situated  on  a  hill  at  the  left, 
whence  the  water  is  conducted  underground  through 
a  pipe  to  the  fountain.  The  jet  will  rise,  in  theory,  to 
the  level  of  the  surface,  but  in  practice  it  falls  short 
of  this,  owing  to  the  friction  at  the  nozzle  of  the  pipe 
and  in  passing  through  the  air,  and  the  weight  of  the 


112 


NATURAL  PHILOSOPHY. 


falling  drops.  It  has  been  thought  that  the  Ro- 
mans knew  nothing  of  this  property  of  liquids,  be- 
cause they  built  immense  stone  aqueducts  a  hundred 
miles  in  length,  spanning  valleys  and  rivers  at  vast 

Fig.  76. 


expense.  Modern  engineers  simply  carry  the  water 
in  pipes  through  the  valley  or  under  the  bed  of  the 
river,  knowing  that  it  will  rise  on  the  opposite  side 
to  its  level.  The  ancients  appear  to  have  under- 
stood this  principle,  but  could  not  make  pipes  capa- 
ble of  resisting  the  pressure. 

Artesian  wells  are  so  named  because  they  have 
been  used  for  a  long  time  in  the  province  of  Artois, 
in  France.  They  were,  however,  employed  by  the 
Chinese  from  early  ages  for  the  purpose  of  procuring 
gas  and  salt  water. 


HYDE  OSTA  TICS.  l  j  3 

Let  A  B  and  C  D  represent  curved  strata  of  clay 
impervious  to  water,  and  K  K  a  layer  of  gravel  and 
fine  sand.  The  rain  falling  on  the  distant  hills 
filters  down  to  C  D,  and  collects  in  this  hollow 

Fig.  77. 


basin.  If  a  well  be  bored  at  H,  as  soon  as  it  reaches 
the  stratum  of  gravel  beneath,  the  water  will  rush 
upward,  under  the  tremendous  lateral  pressure,  to 
the  height  of  the  source,  and  often  spout  high  in 
the  air.  The  well  at  Grenelle,  near  Paris,  is  very 
celebrated.  It  is  at  -the  bottom  of  a  great  chalk- 
basin  which  extends  many  miles  from  the  city. 
It  is  over  1,800  feet  deep  and  furnishes  1,000,000  gal- 
lons daily.  The  wells  of  Chicago,  on  the  level  prai- 
rie, are  about  700  feet  deep/and  discharge  daily 
about  1,250,000  gallons  of  clear  cold  water.  The 
force  with  which  the  water  comes  to  the  surface  in- 
dicates a  head  of  125  feet  above  Lake  Michigan. 
Its  source  must  be  far  away  beyond  Lake  Superior, 


114 


NA  TURAL  PHIL  OS  OPH  Y. 


perhaps  even  beyond  the  Mississippi,  toward  the 
Kocky  Mountains.  Artesian  wells  are  bored  in  the 
sands  of  Sahara ;  gardens  are  planted  and  dates 
flourish  wherever  water  is  supplied.  Brigades  of 
engineers  are  thus  pushing  forward  the  conquest  of 
the  African  desert. 

RULES  FOB  THE  CALCULATION  or  PEESSURE. — 1.  To 
find  tJie  pressure  on  the  bottom  of  a  vessel.  Multiply 
the  area  of  the  base  by  the  perpendicular  height, 
and  that  product  by  the  weight  of  a  cubic  foot  of  the 
liquid. — 2.  To  find  the  pressure  on  the  side  of  a  vessel. 
Multiply  the  area  of  the  side  by  half  of  the  perpen- 
dicular height,  and  that  product  by  the  weight  of  a 
cubic  foot  of  the  liquid. 

The  pressure  on  the  bottom  of  a  cubical  vessel 
full  of  water,  is  the  weight  of  the  water:  on  each 

Fig.  78. 


HYDE  OSTA  TICS.  !  l  ^ 

side,  one-half;  and  on  the  four  sides,  twice  the 
weight ;  therefore  on  the  five  sides,  the  pressure  is 
three  times  the  weight  of  the  water. 

THE  WATER-LEVEL. — The  surface  of  standing  water 
is  said  to  be  level — i.  e.,  horizontal  to  a  plumb- 
line.  This  is  true  for  small  sheets  of  water,  but  for 
larger  bodies  an  allowance  must  be  made  for  the  cir- 
cular figure  of  the  earth.  The  curvature  is  8  inches 
per  mile  ;  22  X  8  inches  =  32  inches  for  two  miles ; 
32  X  8  inches  =  72  inches  for  three  miles,  etc.  The 
spirit-level  is  an  instrument  used  by  builders  for 

Fig.  79. 


levelling.  It  consists  of  a  slightly  curved  glass 
tube  so  nearly  full  of  alcohol  that  it  holds  only  a 
bubble  of  air.  When  the  level  is  horizontal,  the  bub- 
ble remains  at,ihe  centre  of  the  tube. 

SPECIFIC  GRAVITY  is  the  weight  of  a  substance 
compared  with  the  weight  of  the  same  bulk  of  an- 
other substance.  It  is  really  a  method  of  finding  the 
density  of  a  body.  Water  is  taken  as  the  standard* 

*  A  cubic  inch  of  distilled  water  at  a  temperature  of  62°  F., 
with  the  barometer  at  30  inches.  This  standard  weighs  252.456 
grs. :  7,000  grs.  make  a  pound  Avoirdupois  and  58,338  a  gallon. 


n6 


NATURAL  PHILOSOPHY. 


for  solids  and  liquids,  and  air  for  gases.  A  cubic 
inch  of  zinc  weighs  seven  times  as  much  as  a  cubic 
inch  of  water ;  hence  its  specific  gravity  =  7.  A 
cubic  inch  of  carbonic  acid  gas  weighs  1.52  times  as 
much  as  the  same  volume  of  air ;  hence  its  specific 
gravity  =  1.52. 

Buoyant  Force  of  Liquids. — The  cube  a  b  c  d  is  im- 
mersed in  water.     We  see  that  the  lateral  pressure 
Fig.  so.  at  a  is  equal  to  that  at  b,  be- 

cause both  sides  are  at  the 
same  depth;  hence  the  body 
has  no  tendency  toward 
either  side  of  the  jar.  The 
upward  pressure  at  c  is 
greater  than  the  downward 
pressure  at  d,  because  its 
depth  is  greater ;  hence  the 
cube  has  a  tendency  to  rise. 
This  upward  pressure  is  called  the  buoyant  force  of 
the  water.  Its  law,  discovered  by  Archimedes,  is — 

The  buoyant  force  is  equal  to  tJie  weight  of  the  liquid 
displaced.  The  downward  pressure  at  c?  is  the  weight 
of  a  column  of  water  whose  area  is  that  of  the  top 
of  the  cube,  and  whose  perpendicular  height  is  n  d  : 
the  upward  pressure  at  c  is  equal  to  the  weight  of  a 
column  of  the  same  size  whose  perpendicular  height 
is  c  n.  The  difference  between  the  two,  or  the  buoy- 
ant force,  is  the  weight  of  a  bulk  of  water  equal  to 
the  size  of  the  cube. 

The  same  is  shown  in  what  is  called  the  "  cylinder 


HYDROSTA  TICS. 


117 


and  bucket  experiment."  The  cylinder  a  exactly  fits 
in  the  bucket  b.  The  glass  vessel  in  which  the  buck- 
et hangs  is  empty.  The  apparatus  is  balanced  by 
weights  placed  in  the  scale-pan.  Water  is  then 
poured  into  the  glass  vessel.  Its  buoyant  force  Avill 
raise  the  cylinder  and  depress  the  opposite  scale-pan. 

Fig.  81. 


Let  water  be  cautiously  dropped  into  the  bucket ; 
when  it  is  exactly  full,  the  scales  will  balance  again. 
This  proves  that  a  body  in  water  is  buoyed  up  by  a 
force  equal  to  the  weight-  of  the  water  it  displaces. 

To  find  the  specific  gravity  of  a  solid  body  by  a  hy- 


1 1 8  NA  TUBAL  PHIL  OSOPHY. 

drostatic  balance. — Weigh  the  body  in  air,  and  in  wa- 
ter; the  difference  is  the  weight  of  its  bulk  of  water : 
divide  its  weight  in  air  by  its  loss  of  weight  in  water  ; 
the  quotient  is  the  specific  gravity.  Thus,  sulphur 
loses  one-half  its  weight  when  immersed  in  water ; 
hence  it  is  twice  as  heavy  as  water,  and  its  specific 
gravity  =  2. 

To  find  the  specific  gravity  of  a  liquid  by  the  specific- 
gravity  fiask. — This  is  a  bottle  which  holds  exactly 
1,000  grains  of  water.  If  it  will  hold  1,840  grains  of 
sulphuric  acid,  the  specific  gravity  of  the  acid  is  1.84 ; 
if  it  will  hold  13,500  grains  of  mercury,  the  specific 
gravity  of  that  metal  is  13.5. 

To  find  the  specific  gravity  of  a  liquid  by  a  hydrom- 
eter.— This  instrument  consists  of  a  glass  tube,  closed 
at  one  end  and  having  at  the  other  a  bulb  contain- 
ing mercury  or  shot.      A  graduated  scale  is  marked 
Fig.  82.          upon     the    tube.      The    alcoholmeter 
is  so   balanced   as  to   sink  in  pure 
water  to  the  zero  point  at  the  bottom 
of  the   scale.     As  alcohol  is   lighter 
than  water,  the   instrument  will  de- 
scend  for   every  addition   of   spirits 
which  is  made.     The  degrees  of  the 
scale  indicate  the   percentage  of  al- 
cohol.     Instruments  made  in  a  sim- 
ilar manner  are  used  for  determining 
the  strength  of  milk,  acids,  and  solu- 
tions of  various  kinds. 
To  find  the  weight  of  a  given  bulk  of  any  substance. — - 


HYDE OSTA  TICS.  !  l  Q 

Multiply  the  weight  of  one  cubic  foot  of  water  by 
the  specific  gravity  of  the  substance,  and  that  pro- 
duct by  the  number  of    cubic  feet.     Ex. :  What  is 
the  weight  of  three  cubic  feet  of  cork  ?     Solution : 
1,000  oz.  x  .240*  =  240  oz. ;  240  oz.x3  =  720  oz. 
:*    To  find  the  bulk  of  a  given  weight  of  any  substance. — 
Multiply  the  weight  of  a  cubic  foot  of  water  by  the 
specific  gravity  of  the   substance,   and   divide    the 
given  weight  by  that  product.    The  quotient  is  the  re- 
quired bulk  in.  cubic  feet.     Ex. :  What  is  the  bulk  of 
20,000  oz.  of  lead  ?     Solution:  1,000  oz.  x  11.36  * 
=  11,360 ;  20,000  -f-  11,360  =  1.76  +  cu.  ft 
s    To  find  the  volume  of  a  body. — Weigh  it  in  water. 
The  loss  of  weight  is  the  weight  of  the  displaced 
water.     Then,  as  a  cubic  foot  of  water  weighs  1,000 
oz.,  we  can  easily  find  the  bulk  of  water  displaced. 
Ex.  :  A  body  loses  10  oz.  on  being  weighed  in  water. 
The  displaced  water  weighs  10  oz.  and  is  T^  of  a  cu- 
bic foot ;  this  is  the  exact  volume  of  the  body. 
FLOATING  BODIES. — A  very  pretty  experiment  il- 


*  TABLE 

OP  SP] 
21.80 
21.53 
19.34 
13.5 
11.36 
10.5 
8.9 
7.3 
7.81 
7.80 
7.21 

ECIFIC  GRAVITT. 
Flint  Glass,  
Marble,  
Quartz,  
Chalk,.. 

(Seel 
2.76 
2.70 
2.65 
2.65 

lev.  Chem.,  p.  288.) 
Liquids. 
Sulphuric  Acid,  - 
Water  from  the  Dead 
Sea      ... 

1.84 

1.24 
1.03 
1.03 
1. 

.79 
.73 

Platinum,  
Gold 

Mercury 

Lead            

Sulphur,  

2.00 
1.99 
1.83 
1.60 
1.30 
.97 
.93 
.86 
.     .66 
24 

Milk,..          

Silver 

Bone  - 

Cooper 

Phosphorus,..  .. 
Sugar,  

Water 

Tin,      

Absolute  Alcohol,.  ...  .  , 
Ether,  ,.  ,  

Steel,  

Coal,  

Wax,  
Ice,  

Cast-iron,  

Zinc 

7. 
4.43 
3.50 

Potassium,  
Pine  Wood,  
Cork,  

Heavy  Spar,  

Diamond,  

I2O 


NATURAL  PHILOSOPHY. 


Fig.  83. 


lustrative  of  this  subject  is  represented  in  the  cut. 
A  glass  jar  is  half  full  of  water.     An  egg  dropped 

in  it  sinks  directly  to  the 
bottom.     If,  however,   by 
means  of  a  funnel  with  a 
long  tube,  we  pour  a  little 
brine   to  the  bottom  be- 
neath the  fresh  water,  the 
egg  will  gradually  rise.  We 
may  vary  the  experiment 
by  not  dropping  in  the  egg 
until  we  .Lave  half  filled 
the    jar  with   the    brine. 
The  egg  will  then  fall  to 
the  centre,  and  there  float 
like  a  balloon.     Any  solid 
substance     dissolved     in 
water  simply  fills  the  pores 
of  the  water  without  add- 
ing to  its  bulk.     This  in- 
creases its  density  and  buoyant  power.     A  person 
can  therefore  swim  much  more  easily  in  salt  than  in 
fresh  water.     Bayard  Taylor  says  that  he  could  float 
on  the  surface  of  the  Dead  Sea,  with  a  log  of  wood 
for  a  pillow,  as  comfortably  as  if  lying  on  a  spring 
mattress.       Another     traveller    remarks,    that     on 
plunging  in  he  was  thrown  out  again  like  a  cork ; 
and  that  on  emerging  and  drying  himself,  the  crys- 
tals of  salt  which  covered  his  body  made  him  re- 
semble an  "  animated  stick  of  rock-candy." 


HYDROSTATICS. 


121 


A  piece  of  iron  will  float,  if  we  hammer  it  into  a 
vessel  so  that  the  weight  of  the  water  which  it  dis- 
places will  exceed  its  own  weight.  An  iron  ship  wil] 
not  only  float  itself,  but  also  carry  a  heavy  cargo, 
because  it  displaces  a  great  bulk  of  water. 

A  body  floating  in  water  has  its  centre  of  gravity 
at  the  lowest  point.  Herschel  tells  an  amusing 
story  of  a  man  who  attempted  to  walk  on  water  by 
means  of  bulky  cork  boots.  Scarcely,  however,  had 
he  ventured  out  ere  the  law  of  gravitation  seized 
him,  and  all  that  could  be  seen  was  a  pair  of  heels, 
whose  movements  manifested  a  great  state  of  uneas- 
iness in  the  human  appendage  below. 

Fish  are  provided  with  an  air-bladder,  placed  near 
the  spine,  by  means  of  which  they  can  rise  or  sink  at 
pleasure. 

f*racticat  Questions. — 1.  Why  do  housekeepers  test  the  strength  of  lye, 
by  trying  whether  or  not  an  egg  will  float  on  it  ?  2.  How  much  water  will 
it  take  to  make  a  gallon  of  strong  brine  ?  3.  Why  can  a  fat  man  swim  easier 
than  a  lean  one  ?  4.  Why  does  the  firing  of  a  cannon  sometimes  bring  to  the 
surface  the  body  of  a  drowned  person  ?  Ans.  Because  by  the  concussion  it 
shakes  the  body  loose  from  the  mud  or  any  object  with  which  it  is  entangled. 
5.  Why  does  the  body  of  a  drowned  person  generally  come  to  the  surface  of 
the  water,  after  a  time  ?  Ans.  Because  the  gases  which  are  generated  by  de- 
composition in  the  body  render  it  lighter.  6.  If  we  let  bubbles  of  air  pass  up 
through  a  jar  of  water,  why  will  they  become  larger  as  they  ascend  ?  7.  What 
is  the  pressure  on  a  lock  gate  14  feet  high  and  10  feet  wide,  when  the  lock  is 
full  of  water  ?  8.  Will  a  pail  of  water  weigh  any  more  with  a  live  fish  in  it 
than  without  ?  9.  If  the  water  filtering  down  through  a  rock  should  collect 
in  a  crevice  an  inch  square  and  250  feet  high,  opening  at  the  bottom  into  a 
closed  fissure  having  20  square  feet  of  surface,  what  would  be  the  total  pressure 
tending  to  burst  the  rock  ?  10.  Why  can  stones  in  water  be  moved  so  much 
more  easily  than  on  land  ?  11.  Why  is  it  so  difficult  to  wade  in  the  water 
when  there  is  any  current  ?  12.  Why  is  a  mill-dam  or  a  canal  embankment 
small  at  the  top  and  large  at  the  bottom  ?  1 3.  In  digging  canals  and  building 
railroads,  ought  not  the  engineer  to  take  into  consideration  the  curvature  of 
the  earth?  14.  Is  the  water  at  the  bottom  of  the  ocean  denser  than  that  at 
the  surface  ?  15.  Why  does  the  bubble  of  air  in  a  spirit-level  move  as  the 

6 


122  NA  TURA  L  PHIL  OSOPII Y. 

instrument  is  turned?  16.  Cannot  a  swimmer  tread  on  pieces  of  glass  and 
other  sharp  substances  at  the  bottom  of  the  water  without  harm  ?  17.  Will 
a  vessel  draw  more  water  iu  fresh  or  in  salt  water  ?  18.  Will  iron  sink  in  mer- 
cury ?  19.  The  water  in  the  reservoir  in  New  York  is  about  £0  feet  above 
the  fountain  in  the  City  Hall  Park.  What  is  the  pressure  upon  a  single  inch  of 
the  pipe  at  the  latter  point  ?  2O.  Why  does  cream  rise  on  milk  ?  2 1 .  If  a  ship 
founders  at  sea,  to  what  depth  will  she  descend  ?  (*  It  is  a  poetical  thought 
that  ships  may  thus  sink  into  submarine  currents  and  be  carried  hither  and 
thither  with  their  precious  cargoes  of  freight  and  passengers,  on  voyages 
that  know  no  end  and  toward  harbors  that  they  never  reach.)  22.  There  is  a 
story  told  of  a  Chinese  boy  who  accidentally  dropped  his  ball  into  a  deep  hole 
where  he  could  not  reach  it.  He  filled  the  hole  with  water,  but  the  ball  would  not 
quite  float.  He  finally  bethought  himself  of  a  lucky  expedient,  which  was  suc- 
cessful. Can  you  guess  it  ?  23.  Which  has  the  greater  buoyant  force,  water  or 
oil?  24.  What  is  the  weight  of  four  cubic  feet  of  cork?  2  5.  How  many  ounces 
of  iron  will  a  cubic  foot  of  cork  float  in  water  ?  26.  What  is  the  specific  grav- 
ity of  a  body  whose  weight  in  air  is  30  grs.  and  in  water  20  grs.  ?  How  much 
is  it  heavier  than  water  ?  27.  Which  is  heavier,  a  pail  of  fresh  or  one  of  salt 
water?  28.  The  weights  of  a  piece  of  syenite-rock  in  air  and  water  were 
3941.8  grs.  and  2607.5  grs.  Find  its  specific  gravity.  29.  A  specimen  of 
green  sapphire  from  Siam  weighed  in  air  21.45  grs.  and  in  water  1G.33  grs. ;  re- 
quired  its  specific  gravity.  3O.  A  specimen  of  granite  weighs  in  air  534.8  grs. 
and  in  water  334.6  grs.;  what  is  its  specific  gravity  ?  31.  What  is  the  bulk  of 
a  ton  of  iron  ?  A  ton  of  gold  ?  A  ton  of  copper  ?  32.  What  is  the  weight  of  a 
cube  of  gold  4  feet  on  each  side  ?  3  3.  A  cistern  is  12  feet  long,  6  feet  wide,  and 
10  feet  deep ;  when  full  of  water,  what  is  the  pressure  on  each  side  ?  34.  Why 
does  a  dead  fish  always  float  on  its  back  ?  35.  A  given  bulk  of  water  weighs 
62.5  grs.,  and  the  same  bulk  of  muriatic  acid  75  grs.  What  is  the  specific  grav- 
ity of  the  acid  ?  >Ani.  1.2.  36.  A  vessel  holds  10  Ibs.  of  water;  how  much 
mercury  would  it  contain?  37.  A  stone  weighs  70  Ibs.  in  air  and  50  in  wa- 
ter ;  what  is  its  bulk?  38.  A  hollow  ball  of  iron  weighs  10  Ibs. ;  what  must 
be  its  bulk  to  float  in  water  ? 


HYDRAULICS. 

HYDKAULICS  treats  of  liquids  in  motion.  In  this, 
as  in  Hydrostatics,  water  is  taken  as  the  type.  In 
theory,  its  principles  are  those  of  falling  bodies,  but 
they  are  so  modified  by  various  causes,  that  in  prac- 
tice they  cannot  be  relied  upon  except  as  verified  by 
experiment.  The  discrepancy  arises  from  changes 


//  YDS  A  ULICS.  l  2  3 

of  temperature  which  vary  the  fluidity  of  the  liquid, 
from  friction,  the  shape  of  the  orifice,  &c. 

The  velocity  of  a  jet  is  the  same  as  that  of  a  body 
falling  from  the  surface  of  the  ivater. — We  can  see  that 
this  must  be  so,  if  we  recall  two  principles  we  have 
already  learned.  First,  that  "  a  jet  will  rise  to  the 
level  of  its  source ;"  and  second,  that  "  to  elevate  a 
body  to  any  height,  it  must  have  the  same  velocity 
that  it  would  acquire  in  falling  that  distance."  It 
follows,  therefore,  that  the  velocity  of  a  jet  depends 
entirely  on  the  height  of  the  liquid  above  the  orifice, 
and  that  all  liquids  will  issue  with  the  same  velocity 
Nat  the  same  depth.  Molasses  ought  to  flow  with  the 
same  speed  as  mercury,  for  the  same  reason  that  a 
guinea  falls  in  the  same  time  as  a  feather.  The  ap- 
plication of  this  principle  is  of  course  modified  by 
the  temperature,  and  varic  as  other  causes. 

To  find  the  velocity  of  a  jet  of  water. — "We  use  here 
the  4th  equation  of  falling  bodies,  v  =  2  -\fgd,  in 
which  d  is  the  distance  of  the  orifice  below  the  sur- 
face of  the  water.  Ex.  :  The  depth  of  water  above 
the  orifice  is  64  feet ;  required  the  velocity.  Substi- 
tuting 64  for  dj  we  have  v  =  2  ^/IG  x  64  —  64  feet. 

To  find  the  quantity  of  ivater  discharged  in  a  given 
time. — Multiply  the  area  of  the  orifice  by  the  veloci- 
ty of  the  water,  and  that  product  by  the  number  of 
seconds.  Ex. :  What  quantity  of  water  will  be  dis- 
charged in  five  seconds  from  an  orifice  having  an  area 
of  J  a  square  foot,  at  a  depth  of  16  feet  ?  At  that 
depth,  v  =  2  N/16  x  16  =  32  feet  per  second  ;  multi- 


!  2  4  NA  TUEAL  PHIL  OS 0 PHY. 

plying  by  J,  we  have  16  cubic  feet  as  the  amount  dis- 
charged in  one  second  and  80  cubic  feet  in  five  sec- 
onds. In  practice,  however,  it  is  found  that  but  62 
per  cent,  of  this  amount  can  be  realized. 

EFFECT  OF  TUBES. — If  we  examine  a  jet  of  water,  we 
shall  see,  just  outside  the  orifice,  its  size  is  decreased 
to  about  |  that  at  the  opening.  This  is  caused  by  the 
water  producing  cross  currents  as  it  flows  from  dif- 
ferent directions  toward  the  orifice.  If  a  tube  of  a 
length  twice  or  thrice  the  diameter  of  the  opening  be 
inserted,  the  water  adheres  to  the  sides  of  the  tube, 
so  that  there  is  no  contraction,  and  the  flow  is  in- 
creased to  82  per  cent,  of  the  theoretical  amount. 

If  the  tube  be  conical,  and  inserted  with  the  large 
end  in  the  opening,  the  discharge  may  be  increased 
to  92  per  cent. ;  and  strangely  enough,  by  inserting  it 
with  the  smaller  end  next  the  orifice,  the  amount 
exceeds  that  indicated  by  theory  as  much  as  25  per 
cent.  It  seems  in  this  case  to  be  made  so  easy  for 
the  water  to  run,  that  more  is  coaxed  out  than  ought 
to  go.  Long  tubes  or  curves,  however,  by  their 
friction,  largely  diminish  the  flow  of  water.  It  is 
said  that  a  single  right-angle  will  decrease  it  one- 
half,  while  an  inch  pipe  200  feet  long  will  discharge 
only  \  as  much  water  as  one  an  inch  long. 

FLOW  OF  WATER  IN  EIVEKS. — A  fall  of  only  three 
inches  per  mile  is  sufficient  to  give  motion  to  water, 
and  produce  a  velocity  of  as  many  miles  per  hour. 
The  Ganges  descends  but  800  feet  in  1,800  miles.  Its 
waters  require  a  month  to  move  down  this  long 


HYDRAULICS.  I2^ 

inclined  plane.  A  fall  of  3  feet  per  mile  will  make 
a  mountain  torrent.  The  current  moves  more  swiftly 
at  the  centre  than  near  the  shores  or  bottom  of  a 
channel,  since  there  is  less  friction. 

WATER-WHEELS  are  machines  for  using  the  force 
of  falling  water.  By  means  of  bands  or  cog-wheels 
the  motion  of  the  wheel  is  conducted  from  the  axle 
into  the  mill.  The  principle  is  that  of  a  lever  with 
the  P  acting  on  the  short  arm.  In  this  way,  the 
movement  of  the  slow  creaking  axle  reappears  in  the 
swiftly  buzzing  saw  or  flying 
spindle.  "Water-wheels  are  of 
four  classes — The  Overshot,  Un- 
dershot, Breast,  and  Turbine 
wheels.  The  Overshot-iuheel  has 
on  its  circumference  a  series  of 
buckets  which  receive  the  water 
as  it  flows  out  of  a  sluice,  C.  The 
buckets  are  so  made  as  to  hold  the  water  as  they 
descend  on  one  side,  and  to  empty  it  as  they  come 
up  on  the  other.  Overshot-wheels  are  valuable 
where  a  great  fall  can  be  secured,  since  they  require 
but  little  water.  They  are  made  of  great  size.  One 
at  Cohoes,  N.  Y.,  is  96  feet  high.  If  P  denotes  the 
weight  of  the  water  and  d  the  distance  it  falls,  then 
the  total  force  =  Pc£.  Of  this  amount  80  per  cent, 
can  be  secured  in  the  best  wheels  of  this  and  the 
third  class. 

The  Undershot-wheel,  instead  of  buckets  has  merely 
projecting  boards  or  floats,  which  receive  the  force 


126  NA  TURA  L  PHIL  OSOPH  Y. 

of  the  current.  It  is  of  use  where  there  is  little  fill 
and  a  large  quantity  of  water.  It  is  said  to  utilize 
only  20  per  cent,  of  the  force  of  the  water. 

'  Pig.  85.  Fig.  86. 


TJie  Breast-ivheel  is  a'  medium  between  the  two 
before  named,  as  may  be  seen  in  Fig.  86. 

The  Turbine-wheel  differs  essentially  from  the 
others  named.  It  is  placed  horizontally,  and  is  en- 
tirely immersed  in  the  water.  In  the  figure,  C  is 
the  dam  and  D  A  the  spout  by  which  the  water  is 
furnished  to  the  wheel.  E  is  a  scroll-like  casing  en- 
circling the  wheel,  and  open  at  the  centre  above  and 
below.  The  axis  of  the  wheel  is  the  cylinder  /',  from 
which  radiate  plane-floats  against  which  the  water 
strikes.  To  confine  the  water  at  the  top  and  the  bot- 
tom is  a  circular  disk  attached  to  the  cylinder  and  the 
floats.  In  these  disks  are  the  swells  for  discharging  the 
water.  They  project  above  and  below,  as  seen  in  the 
figure.  They  commence  near  the  cylinder,  and  swell- 
ing outward  scroll-shaped,  form  openings  curved 
toward  the  cylinder,  thus  emptying  the  water  in  a 
direction  opposite  to  that  in  which  it  enters  the 


HYDRA  ULICS. 


127 


wheel.  This  form  utilizes  as  high  as  90  per  cent,  of 
the  force.  F  is  a  band- wheel  which  conducts  the  power 
to  the  machinery.  The  principle  of  the  turbine  is 

Fig.  87. 


that  of  the  unbalanced  pressure  of  a  column  of  wa- 
ter. It  is  finely  illustrated  in  the  old-fashioned 
Barker's  Mill  or  Reaction  Wheel.  This  consists  of 
an  upright  cylinder  with  horizontal  arms,  on  the 
opposite  sides  of  which  are  small  apertures.  It  rests 
in  a  socket,  so  as  to  revolve  freely.  Water  is  sup- 
plied from  a  tank  above.  If  the  openings  in  the 
arms  are  closed,  when  the  cylinder  is  filled  with 
water  the  pressure  will  be  equal  in  all  directions  and 


128 


NATURAL  PHILOSOPHY. 


Fig.  88. 


the  machine  will  be   at   rest.     If  now  we  open  an 
aperture,  the  pressure  is  relieved  on  that  side,  and 

the  arm  flies  back- 
ward with  the  un- 
balanced pressure  of 
the  column  of  water 
above. 

WAVES  are  pro- 
duced by  the  friction 
of  the  wind  against 
the  surface  of  the 
water.  A  light  wind 
forms  merely  ripples ; 
these  increase  out  in 
the  open  sea,  as 
wave  is  raised  upon 
wave,  until  they  be- 
come great  billows 
which  constantly 
surge  to  and  fro,  so 
that  the  sea  is  never 
at  rest.  The  wind 
raises  the  particles  of 
water  and  gravity 
draws  them  back  again.  They  thus  vibrate  up  and 
down,  but  do  not  advance.  The  forward  movement 
of  the  wave  is  only  an  illusion.  The  form  of  the 
wave  progresses,  but  not  the  water  of  which  it  is 
composed,  any  more  than  the  thread  of  a  screw 
which  we  turn  in  our  hand ;  or  the  undulations  of  a 


HYDRAULICS. 


I29 


rope  or  carpet  which  is  being  shaken ;  or  the  stalks 
of  grain  which  bend  in  billows  as  the  wind  sweeps 
over  them.  If  we  watch  a  buoy  in  the  harbor  or 
a  body  floating  on  the  surface  of  water,  we  shall 
see  that  it  moves  forward  on  the  crest  of  each 
wave  through  a  few  feet  or  inches,  according  to 
the  length  of  the  wave;  then  stops,  moves  back- 
ward in  the  hollow ;  stops,  and  again  moves  forward 
as  before  on  the  crest  of  the  next  wave.  The  mole- 
cules of  water  vi- 
brate to  and  fro  in 
an  elliptical  path. 
Thus,  let  the  figure 
represent  two  suc- 
cessive wave-crests  and  the  hollow  between.  While 
the  whole  wave  moves  from  the  position  A  B  to  that 
of  C  D,  the  molecules  of  water  only  move  backward 
or  forward  through  a  distance  A  B  or  C  D  ;  forward 
on  the  crest  of  the  wave  and  backward  in  the  hollow 
as  shown  by  the  arrows.  The  velocity  of  the  particles 
may  be  much  slower  than  that  of  the  progressive 
motion  of  the  wave.  It  is  said  that  in  an  earthquake, 
the  velocity  of  the  particles  of  the  shaken  ground  is 
often  only  three  feet  per  second,  while  the  earth-wave 
moves  across  the  country  at  the  rate  of  3,000  feet  per 
second. 

Near  the  shore  the  character  of  waves  is  some- 
what changed.  The  oscillations  are  shorter,  and  as 
the  waves  do  not  balance  those  in  the  deeper  water, 
they  are  forced  forward  till  the  lower  part  of  each  wave 

6* 


!^0  NATURAL  PHILOSOPHY. 

is  checked  by  the  friction  on  the  sandy  beach,  and 
the  upper  part  curls  over  and  falls  beyond.  The  size 
of  "  mountain  billows"  has  been  much  exaggerated. 
The  ocean  is  probably  undisturbed  below  the  depth 
of  30  feet.  The  highest  wave,  from  the  deepest 
"trough"  to  the  very  topmost  "crest,"  is  only  43  feet. 
The  corresponding  parts  of  different  waves  are 
termed  like  phases.  The  distance  between  two  like 
phases,  or  between  the  crests  of  two  succeeding 
waves,  is  called  a  wave-length.  Opposite  phases  are 
those  parts  which  are  vibrating  in  different  direc- 
tions, as  the  point  midway  in  the  front  of  one  wave 
and  another  midway  in  the  rear  of  the  next  wave. 

A  tide- wave  may  be  setting  steadily  toward  the 
west ;  waves  from  distant  storms  may  be  moving  upon 
this ;  and  above  all,  ripples  from  the  breeze  then  blow- 
ing may  diversify  the  surface.  These  different  systems 
will  each  be  entirely  distinct,  yet  the  joint  effect  may 
be  very  peculiar.  If  any  two  systems  exactly  coin- 
cide with  like  phases, — the  crest  of  one  meeting  the 
crest  of  the  other,  and  the  furrow  of  one  meeting  the 
furrow  of  the  other, — the  resulting  wave  will  have  a 
height  equal  to  the  sum  of  the  two.  If  any  two  coin- 
cide with  opposite  phases, — the  hollow  of  one  strik- 
ing the  crest  of  another, — the  height  will  be  the  dif- 
ference of  the  two.  Thus,  if  in  two  systems  having  the 
same  wave-length  and  height,  one  is  exactly  half 
a  length  behind  the  other,  they  will  mutually  destroy 
each  other.  This  is  termed  the  interference  of  waves. 
The  manner  in  which  different  waves  move  among 


HYDRAULICS. 


and  upon  each  other,  is  seen  by  dropping  a  handful 
of  stones  in  water  and  watching  the  waves  as  they 

?ig.  90. 


circle  out  from  the  various  centres  in  ever-widening 
curves.  In  the  figure  is  shown  the  beautiful  appear- 
ance these  waves  present  when  reflected  from  the 
sides  of  a  vessel. 

The  application  of  these  principles  in  Sound  and 
Light  will  be  found  very  important. 

f*raclicat  Questions— \.  How  much  more  water  can  be  drawn  from  a 
faucet  8  feet  than  from  one  4  feet  below  the  surface  of  the  water  in  a  cistern  ? 
2.  How  much  water  will  be  discharged  per  second  from  a  short  pipe  hav 
ing  a  diameter  of  4  inches  and  a  depth  of  43  feet  below  the  surface  of  the  wa- 
ter ?  3.  When  we  pour  molasses  from  a  jug,  why  is  the  stream  so  much 
larger  near  the  nozzle  than  at  some  distance  from  it  ?  4.  Ought  a  faucet  to 
extend  into  a  barrel  beyond  the  staves  ?  5.  What  would  be  the  effect  if  both 
openings  in  one  of  the  arms  of  Barker's  Mill  were  on  the  same  side  ? 


132 


NATURAL  PHILOSOPHY. 


PNEUMATICS. 

PNEUMATICS  treats  of  the  general  properties  and  the 
pressure  of  gases.  Since  the  molecules  move  among 
each  other  more  freely  even  than  those  of  liquids,  the 
conclusions  at  which  we  have  arrived  with  regard  to 
transmission  of  pressure,  buoyancy  and  specific  gravity 
apply  also  to  gases.  Its  principles  obtain  in  all  gas- 
eous bodies,  but  as  air  is  the  most  abundant  gas,  it  is 
taken  as  the  type  of  the  class,  as  water  is  of  liquids. 
THE  AIK-PUMP  is  shown  in  its  essential  features 
in  Fig.  91.  A  is  a  glass  receiver  standing  on  an  oiled 
pump-plate.  The  tube  D,  connecting  the  receiver 
with  the  cylinder,  is  closed  by  the  valve  E  opening 
upward.  There  is  a  second  valve,  P,  in  the  piston, 
also  opening  upward.  Suppose  the  piston  is  at  the 

bottom  and  both  yalves 
shut.  Let  it  now  be  raised, 
and  there  will  be  a  vacuum 
produced  in  the  cylinder ; 
the  expansive  force  of  the 
atmosphere  in  the  receiver 
will  open  the  valve  E  and 
drive  the  air  through  to  fill 
this  empty  space.  "When  the  piston  descends,  the 
valve  E  will  close,  while  the  valve  P  will  open, 
and  the  air  will  pass  up  above  the  piston.  On  ele- 
vating the  piston  a  second  time,  this  air  is  removed 


Fig.  91. 


PNEUMATICS. 
Fig.  92. 


133 


The  Air-pump. 

from  the  cylinder,  while  the  air  from  the  receiver 
passes  through  as  before.     At  each  stroke  a  portion 
of  the  atmosphere  is  drawn  off ;  but          Fig.  93. 
the  expansive  force  becomes  less  and 
less,  until  finally  it  is  not  sufficient 
even  to  lift  the  delicate  valves.     For 
this  reason  a  perfect  vacuum  cannot 
be  obtained. 

PROPERTIES  OF  THE  AIR. — Weight. — 
Exhaust  the  air  from  a  flask  which 
holds  100  cubic  inches,  and  then  bal- 
ance it  accurately.  If  now  we  turn 
the  stop-cock,  the  air  will  rush  in  with 


134 


NATURAL  PHILOSOPHY. 


a  whizzing  noise  and  the  flask  will  descend.  We  shall 
have  to  add  about  31  grains  to  restore  the  equipoise. 
Elasticity  and  compressibility. — These  properties  are 
shown  in  the  common  pop-gun.  We  compress  the 
atmosphere  in  the  barrel  until  the  elastic  force  be- 
comes so  great  as  to  drive  out  the  stopper  with  a 
loud  report.  As  we  crowd  down  the  piston  we  feel 
the  elasticity  of  the  air  yielding  to  our  strength,  like 
a  cushion  or  a  bent  spring. 

Fig.  94. 


The  bottle-imps,  or  Cartesian  divers,  illustrate  the 
game  properties.      Fig.  94  represents  a  very  simple 


PNEUMA  TICS.  l  ^  5 

form  of  this  experiment.  The  cover  of  a  common 
fruit-jar  is  fitted  with  a  small  tin  tube,  which  is  in- 
serted into  a  syringe-bulb.  The  jar  is  filled  with 
water  and  the  divers  placed  within.  These  are  hol- 
low images  of  glass,  having  each  a  small  opening  at 
the  end  of  the  curved  tail.  If  we  squeeze  the  bulb, 
the  air  will  be  forced  into  the  jar  and  the  water  will 
transmit  the  pressure  to  the  air  in  the  image.  This 
being  compressed,  the  water  will  enter,  and  the  spe- 
cific gravity  being  increased,  the  diver  will  descend. 
On  relaxing  the  grasp  of  the  hand  on  the  bulb,  the 
air  will  return  into  it,  the  air  in  the  image  will  ex- 
pand by  its  elastic  force  driving  out  the  water,  and 
the  diver,  thus  lightened  of  his  ballast,  will  ascend. 
The  nearer  the  image  is  to  the  bottom,  the  less  force 
will  be  required  to  move  it.  With  a  little  care  it  can 
be  made  to  respond  to  the  slightest  pressure,  and 
will  rise  and  fall  as  if  instinct  with  life.  This  experi- 
ment shows  also  the  buoyant  force  of  liquids,  their 
transmission  of  pressure  in  every  direction,  the  in- 
crease of  the  pressure  in  proportion 
to  the  depth,  and  the  principle  of 
Barker's  Mill. 

Expansibility. — Let  a  well-dried 
bladder  be  partly  filled  with  air 
and  tightly  closed.  Now  place  it 
under  the  receiver  and  exhaust  the 
air.  The  air  within  the  bladder  ex- 
panding will  swell  arid  oftentimes  burst  it  into  shreds. 

Take  two  bottles  partly  filled  with  colored  water. 


NATURAL  PHILOSOPHY. 


Pig  97 


Let  a  bent  tube  be  inserted  tightly  in  A  anc| 
loosely  in  B.  Place  this  apparatus  under  the  re- 
ceiver and  exhaust  the  air.  The  expansive  force  of 
Pig.  %.  the  air  in  A  will  drive  the  water 

over  into  B.     On  readmitting  the 
air  into  the  receiver,  the  pressure 
will  return  the  water  into  A.     It 
may  thus   be   driven    from    one 
bottle  to  the  other  at  pleasure. 
PRESSURE   OF  THE  AIR.  —  If  we  place   the  hand- 
glass on  the  plate  of  the  Air-pump,  covering  it  with 
one  nand,  on  exhausting  the  air 
we  shall  soon  find  the  pressure  to 
become  painful.     Tie  over  one 
end  of  the  glass  a  piece  of  well- 
soaked    bladder.     When    thor- 
oughly dry,  exhaust  the  air  from 
it  as  before,  and  the  membrane 
will  burst  with  a  sharp  report. 

The  Magdeburg  Hemispheres 
are  named  from  the  city  in  which 
Otto   Guericke,  their  inventor,  resided.     They  con- 
sist of  two  small  brass  hemispheres,  which  fit  closely 
Fig.  98.  together,  but  may  be 

separated  at  pleas- 
ure. If,  however,  the  ; 
air  be  exhausted 
from  within,  the 
strength  of  several 
persons  will  be  required  to  pull  them  apart.  No 


PNEUMATICS. 


137 


Fig.  99. 


matter  in  what  position  the  hemispheres  are  held, 
we  shall  find  the  pressure  the  same. 

Upward  Pressure  of  the  Air. 
— Fill  a  tumbler  with  water,  and 
then  lay  a  sheet  of  paper  over 
the  top.  Quickly  invert  the 
glass,  and  the  water  will  be  sup- 
ported by  the  upward  pressure  of 
the  air. 

Within  the  glass  cylinder 
shown  in  Fig/  100  there  is  a  piston  working  air- 
tight. Connect  C  with  the  pump  by  means  of  a 
rubber  tube  and  exhaust  the 
air.  The  weight  will  leap 
up  as  if  caught  by  a  spring. 

Buoyant  Force  of  the  Air. 
—The  principle  of  Archi- 
medes holds  true  in  gases  as 
in  liquids.  Illustrations  of 
this  abound  in  common  life. 
Smoke  and  other  light  sub- 
stances float  in  the  air,  as 
wood  does  in  water,  because 
they  are  lighter  and  are 
buoyed  with  a  force  equal  to 
the  weight  of  the  air  they  displace.  In  Fig.  101 
we  have  a  hollow  sphere  of  copper,  which  is  ex- 
actly balanced  in  the  air  by  a  solid  lead  weight, 
but  instantly  falls  on  being  placed  under  the  re- 
ceiver and  the  air  exhausted.  This  shows  that  its 


NATURAL  PHILOSOPHY. 


weight  was  partly  sustained  by  the  buoyant  force  of 
the  air. 

The.  pressure  of  the  air  sustains  a  column  of  mercury 


Fig.  101. 


Fig.  102. 


30  inches  high,  of  water  34 
feet  high,  and  is  15  Ibs.  per 
square  inch. 

Take  a  strong  glass 
tube  about  three  feet  in 
length,  and  tie  over  one 
end  a  piece  of  well-soaked 
bladder.  "When  thorough- 
ly dry,  fill  the  tube  with 

mercury,  and  invert  it  in  a  cup  of  the  same  liquid. 
The  mercury  will  sink  to  a  height  of  about  30  inches. 
If  the  area  of  the  tube  be  one  inch,  this  amount  of 
the  metal  will  weigh  about  15  Ibs.  The  weight  of 
the  column  of  mercury  is  equal  to  the  downward 
pressure  on  each  square  inch  of  the  surface  of  the 


PNEUMATICS. 


mercury  in  the  cup.  Hence  we  conclude  that  the 
pressure  of  the  atmosphere  is  15  Ibs.  per  square 
inch,  and  will  balance  a  column  of  mercury  30 
inches  high.  As  water  is  13^  times  lighter  than 
mercury,  it  is  evident  that  the  same  pressure  would 
balance  a  column  of  that  liquid  13J  times  higher, 
or  3$|  feet.  On  account  of  the  unwieldy  length 
;uxf  the  tube  required  to  exhibit  the  column  of 
water,  it  is  not  easy  to  verify  this  last  statement. 
It  may,  however,  be  prettily  illustrated  in  the  fol- 
lowing manner.  Pour  on  the  mercury  in  the  cup 
(Fig.  102)  a  little  water  colored  with  red  ink.  Now 
raise  the  end  of  the  tube  carefully  above  the  surface 
of  the  metal,  but  not  above  that  of  the  water  which 
will  immediately  rise  in  the  tube,  the  mercury  passing 
down  in  beautifully  beaded  globules.  The  mercu- 
rial column  was  only  30  inches  high,  while  the  water 
will  entirely  fill  the  tube.  Finish  the  experiment  by 
puncturing  the  bladder  with  a  pin,  when  the  water 
will  instantly  fall  to  the  cup  below. 

The  pressure  of  the  air  varies.  —  We  live  on  the  bed 
of  an  aerial  ocean  whose  invisible  tides  surge  around 
us  on  every  side.  More  restless  than  the  sea,  its 
waves  beat  to  and  fro,  stirred  by  a  multitude  of  causes. 
Changes  of  temperature,  moisture,  <fec.,  constantly 
vary  the  weight  of  the  air,  and  consequently  change 
the  height  of  the  column  of  liquid  which  it  can  sup- 
port. There  is  also  a  diurnal  variation,  due  to  the 
heat  of  the  sun,  —  slight  indeed,  yet  so  marked  that 
Humboldt  says  that  the  play  of  the  mercurial  column 


140 


NATURAL  PHILOSOPHY. 


could  be  used  to  indicate  the  hour  of  the  day.  The 
pressure  of  the  air  increases  with  the  depth.  Hence, 
in  a  valley  its  weight  is  greater  than  on  a  mountain. 

Fig.  103.  Fig.  104. 


The  figures  given  in  the  last  paragraph  apply  only 
to  the  level  of  the  sea  and  the  temperature  of  60°  F. 
They  are  an  average  of  all  the  variations,  and  are 
considered  the  standard  for  reference. 


PNEUMATICS.  I4I 

Mariotte's  Law. — Fig.  103  represents  a  long^bent 
glass  tube  with  the  end  of  the  short  arm  closed. 
Pour  mercury  into  the  long  arm  until  it  rises  to  the 
point  marked  zero.  It  stands  at  the  same  height  in 
both  arms,  and  there  is  an  equilibrium.  The  air 
presses  on  the  mercury  in  the  long  arm  with  a  force 
equal  to  a  column  of  mercury  30  inches  jig.ios. 
high,  and  the  elastic  force  of  the  air  confined 
in  the  short  arm  is  equal  to  the  same  amount. 
Let  us  now  pour  additional  mercury  into  the 
long  arm  until  it  stands  at  30  inches.  (Fig. 
104.)  We  have  evidently  doubled  the  pres- 
sure. If  we  look  at  the  short  arm,  we  shall 
find  that  the  air  is  condensed  to  one-half 
its  former  dimensions,  and  of  course  the 
expansive  force  must  be  doubled.  We  there- 
fore conclude  that  the  elasticity  of  a  gas  in- 
creases and  the  volume  diminishes  in  proportion 
to  the  pressure  upon  it. 

THE  BAROMETER  is  an  instrument  for 
measuring  the  pressure  of  the  air.  It  con- 
sists essentially  of  the  tube  and  cup  of  mer- 
cury shown  in  Fig.  102.  A  scale  is  attached 
for  convenience  of  reference.  The  barom- 
eter is  used  (1)  to  indicate  the  weather,  and 
(2)  to  measure  the  height  of  mountains. 

It  does  not  absolutely  foretell  the  charac- 
ter of  the  weather.     It  simply   shows  the 
varying  weight  of  the  air,  from  which  we  must  draw 
our  own  conclusions.     A  continued  rise  of  the  mer- 


142 


NATURAL  PHILOSOPHY. 


cury  indicates  fair  weather,  and  a  continued  fall,  foul 
weather. 

Since  the  pressure  diminishes  as  one  ascends  above 
the  level  of  the  sea,  the  observer  ascertains  the  fall 
of  the  mercury  in  the  barometer,  and  the  tempera- 
ture by  the  attached  thermometer  ;  and  then,  by  ref- 
'erence  to  carefully  prepared  tables,  easily  determines 
the  height. 

Water-barometer. — Mercury  is  used  for  filling  the 
barometer  because  of  its  weight  and  its  low  freezing- 
point.  Water  would  require  a  tube  about  34  feet  in 
length.  It  is  said  that  the  first  barometer  was  filled 
with  that  liquid.  The  inventor,  Otto  Guericke,  a 
wealthy  burgomaster  of  Magdeburg,  Saxony,  erected 
a  tall  tube  reaching  from  a  cistern  in  the  cellar  up 
through  the  roof  of  his  house.  A  tall  wooden  image 
—life-size — was  placed  within  the  tube,  floating  upon, 
the  water.  On  fine  days,  this  novel  weather-prophet 
would  rise  above  the  roof-top  and  peep  out  upon  the 
queer  old  gables  of  that  ancient  city,  while  in  foul 
weather  he  would  retire  to  the  protection  of  the  gar- 
ret. The  accuracy  of  these  movements  attracted  the 
attention  of  the  neighbors.  Finally,  in  their  inno- 
cency,  becoming  suspicious  of  Otto  GLuericke's  piety, 
they  openly  accused  him  of  being  in  league  with  the 
devil.  So  the  offending  philosopher  relieved  this 
wicked  wooden  man  from  longer  dancing  attendance 
upon  the  weather,  and  the  staid  old  city  was  once 
more  at  peace. 

PUMPS. — Two  varieties  are  in  common  use.  These 
are  the  Lifting  and  the  Forcing  pump. 


PNEUMATICS. 


143 


TJie  Lifting-pump  contains  two  valves  opening 
upward — one,  a,  at  the  top  of  the  suction-pipe,  B; 
the  other,  c,  in  the  piston.  Suppose  the  handle  to 
be  raised,  the  piston  at  the  bottom  of  the  cylinder 


Fig.  106. 


and  b:>th  valves  closed.  Now  depress  the  pump- 
hand]^  and  elevate  the  piston.  This  will  produce  a 
partial  vacuum  in  the  suction-pipe.  The  pressure  of 
the  air  on  the  surface  of  the  water  below  will  force 
the  water  up  the  pipe,  open  the  valve,  and  fill  the 
chamber,  as  seen  in  the  first  figure.  Let  the  pump- 
handle  be  elevated  again,  and  the  piston  depressed. 
The  valve  a  will  now  close,  the  valve  c  will  open  and 


144 


NATURAL  PHILOSOPHY. 


Fig.  107. 


the  water  will  flow  through  it  above  the  piston,  as  in 
the  second  figure.  When  the  pump-handle  is  low- 
ered the  second  time  and  the  piston  elevated,  the  wa- 
ter is  lifted  up  to  the  spout,  whence  it  flows  out ; 
while  at  the  same  time  the  lower  valve  opens  and 
the  water  is  forced  up  from  below  by  the  pressure 
of  the  air,  as  in  the  third  figure. 

If  the  valves  and  piston  were  fitted  air-tight,  the 
water  could  be  raised  34  feet  (more  exactly  13  J  times 
the  height  of  the  barometric  column)  to  the  lower 
valve,  but  owing  to  various  imperfections  it  com- 
monly reaches  only  28  feet. 
For  a  similar  reason  we  some- 
times find  a  dozen  strokes  neces- 
sary to  "  bring  water." 

The  Force-pump  has  no  valve 
in  the  piston.  The  water  rises 
above  the  lower  valve  as  in  the 
lifting-pump.  "When  the  piston 
descends,  the  pressure  opens 
the  valve  O  and  forces  the  wa- 
ter up  the  pipe  D.  This  pipe 
may  be  made  of  any  length, 
and  thus  the  water  driven  to 
any  height. 

The   Fire-engine   consists    of 
two   force-pumps   with   an  air- 
^==____  chamber.     The  water  is  driven 

by  the  pistons  m,  n,  alternately 
into  the  chamber  R,   whence   the   air,  by  its  ex- 


PNEUMATICS. 


145 


pansive  force,  throws  it  out  in  a  continuous  stream 
through  the  hose-pipe  attached  at  Z. 


Fig.  108. 


THE  SIPHON  consists  simply  of  a  tube  bent  in  the 
form  of  the  letter  U,  having  one  arm  longer  than  the 
other.  We  insert  the  short  arm"  in  the  water,  and 
then  applying  the  mouth  to  the  other,  exhaust  the 
air.  The  water  will  immediately  begin  to  flow  from 
the  long  arm,  and  continue  until  the  lower  end  of  the 
short  arm  is  uncovered,  or  until  the  water  in  the  two 
vessels  comes  to  the  same  level.  A  very  instructive 
variation  of  this  experiment  may  be  given  if  we  color 
the  water  with  red  ink,  and  then  allow  it  to  run  from 
one  tumbler  into  another  until  just  before  the  flow 


146 


NATURAL  PHILOSOPHY. 


would  cease  ;  then  quickly  elevate  the  vessel  con- 
taining the  long  arm,  carefully  keeping  both  ends  of 
the  siphon  under  the  water,  when  the  flow  will  set 
back  to  the  first  vessel.  Thus  we  may  alternate, 


Fig.  109. 


backward  and  forward,  until  we  see  clearly  that  the 
water  flows  always  to  the  lower  level  from  the  long 
arm,  and  ceases  whenever  the  water  in  the  two  vessels 
reaches  the  same  level. 

The  Theory  of  the  Siphon. — The  pressure  of  the  air 
at  b  holds  up  the  column  of  water  a  b,  and  the  up- 
ward pressure  is  the  weight  of  the  air  less  the  weight 
the  column  of  water  a  1).  The  upward  pressure 
at  d  is  the  weight  of  the  air  minus  the  weight  of  the 
column  of  water  c  d.  Now  c  d  is  less  than  a  I, 


PNEUMA  TICS. 


147 


Fig.  110. 


therefore  the  pressure  at  d  is  greater  than  that  at  bt 
and  the  water  in  the  tube  is  driven  toward  the  longer 
arm  by  a  force  equal  to  the  difference  between  the 
two  arms, 

THE  PNEUMATIC  INKSTAND  is  filled  by  pouring  in. 
the  ink  when  the  bottle  is  tipped  so  that  the  nozzle 
is  at  the  top.  The  pressure 
of  the  air  will  then  hold  the 
ink  in  the  stand.  When  it 
is  used  below  the  level  of  o, 
a  bubble  of  air  passes  in, 
forcing  the  ink  into  the  noz- 
zle as  desired. 

THE  ANCIENTS  noticing 
how  the  air  rushes  in  to 
fill  any  empty  space,  ex- 
plained the  fact  by  saying, 
"  Nature  abhors  a  vacuum."  This  principle  an- 
swered the  purpose  of  philosophers  even  in  modern 
times.  In  the  17th  century,  when  workmen  were 
employed  by  the  Duke  of  Tuscany  to  dig  a  very 
deep  well  near  Florence,  they  found  to  their  sur- 
prise that  the  water  would  not  rise  in  the  pump  as 
high  as  the  lower  valve.  They  applied  in  their 
dilemma  to  Galileo.  The  old  philosopher  replied — 
half  in  jest  we  hope,  certainly  he  was  half  in  earnest — 
"  Nature  does  not  abhor  a  vacuum  beyond  34  feet." 

THREE  OPPOSING  FORCES  ACT  UPON  THE  AIR — viz: 
Gravity,  which  binds  it  to  the  earth,  and  the  Centrif- 
ugal and  the  Repellant  [heat]  forces,  which  tend  to 


I4g  NATURAL  PHILOSOPHY. 

hurl  it  off  into  space.  Under  the  action  of  the  latter 
forces  the  atmosphere,  like  a  great  bent  spring,  is 
ready  to  bound  awa j  at  the  first  opportunity ;  but  the 
attraction  of  the  earth  holds  it  firmly  in  its  place. 

HEIGHT  AND  DENSITY  OF  THE  AIK. — Fifty  miles 
has  been  taken  as  the  extreme  limit  of  the  atmos- 
phere. The  latest  investigations,  however,  indicate 
that  there  is  an  extremely  rarefied  air  at  the  height 
of  perhaps  500  miles.  Its  density  rapidly  dimin- 
ishes as  we  ascend.  At  the  height  of  3J  miles  it  is 
but  one-half  that  at  the  sea-level.  At  40  miles  the 
atmosphere  is  rare  as  in  the  vacuum  of  an  air-pump. 


Practical  Questions. — 1.  Why  must  we  make  two  openings  in  a  barrel 
of  cider  when  we  tap  it  ?  2.  What  is  the  weight  of  10  cubic  feet  of  air  ?  3. 
What  is  the  pressure  of  the  air  on  1  square  rod  of  land  ?  4.  What  is  the  pres- 
sure on  a  pair  of  Magdeburg  hemispheres  4  inches  in  diameter?  5.  How 
high  a  column  of  water  can  the  air  sustain  when  the  barometric  column  stands 
at  28  inches  ?  6.  If  we  should  add  a  pressure  of  two  atmospheres  (30  Ibs.  to 
the  square  inch),  what  would  be  the  volume  of  100  cubic  inches  of  common 
air  ?  7.  If,  while  the  water  is  running  through  the  siphon,  we  quickly  lift  the 
long  arm,  what  is  the  effect  on  the  water  in  the  siphon  ?  If  we  lift  the  entire 
siphon?  8.  When  the  mercury  stands  at  29£  inches  in  the  barometer,  how 
high  above  the  surface  ot  the  water  can  we  place  the  lower  pump-valve  ?  9. 
Why  cannot  we  raise  water  by  means  of  a  siphon  to  a  higher  level  ?  10. 
If  the  air  in  the  chamber  of  a  fire-engine  be  condensed  to  Vie  its  former  bulk, 
•what  will  be  the  pressure  due  to  the  expansive  force  of  the  air  on  every  square 
inch  of  the  air-chamber  ?  11.  What  causes  the  bubbles  to  rise  to  the  surface 
when  we  put  a  lump  of  loaf-sugar  in  hot  tea  ?  1 2.  To  what  height  can  a  bal- 
loon ascend  ?  What  weight  can  it  lift  ?  13.  The  rise  and  fall  of  the  barometric 
column  shows  that  the  air  is  lighter  in  foul  and  heavier  in  fair  weather.  Why 
is  this  ?  Ans.  In  fair  weather  the  moisture  of  the  air  is  an  invisible  vapor 
mingled  with  it  and  adding  to  its  pressure,  while  in  foul  weather  the  vapor  is 
separated  in  the  form  of  clouds.  14.  When  smoke  ascends  in  a  straight  line 
from  chimneys,  is  it  a  proof  of  the  rarity  or  density  of  the  air  ?  15.  Why  do 
We  not  feel  the  heavy  pressure  of  the  air  upon  our  bodies  ?  16.  Is  a, bottle 
empty  when  filled  with  air  ?  17.  Why  is  it  so  tiresome  to  walk  in  miry  clay  ? 
Ans.  Because  the  upward  pressure  of  the  air  is  removed  from  our  feet.  1 8. 
How  does  the  variation  in  the  pressure  of  the  air  affect  those  who  ascend 
lofty  mountains  ?  Who  descend  in  diving-bells  ?  19.  Explain  the  theory  of 
"  sucking  cider"  through  a  straw. 


n 


ound. 


"  Science  ought  to  teach  us  to  see  the  invisible  as  well  as  the  visible  in 
nature :  to  picture  to  our  mind's  eye  those  operations  that  entirely  elude  the 
eye  of  the  body  ;  to  look  at  the  very  atoms  of  matter,  in  motion  and  in  rest, 
ftnd  to  follow  them  forth  into  the  world  of  the  senses." — TYNDALL. 


ACOUSTICS. 

Acoustics  treats  of  sound.* 

SOUND  is  PRODUCED  BY  VIBRATIONS. — By  lightly 
tapping  a  receiver  or  even  a  glass  fruit-dish,  you  can 
see  that  the  sides  are  thrown  into  motion.  Fill  a 
goblet  half  full  of  water,  and  wetting  your  finger,  rub 
it  lightly  around  the  upper  edge  of  the  glass.  The 
sides  will  vibrate,  and  tiny  waves  corresponding  to 
these  movements  will  ripple  the  surface  of  the  water. 
The  vibrations  of  a  tuning-fork  are  very  distinct. 
Hold  a  card  close  to  its  prongs,  and  you  can  hear 

*  The  term  sound  is  used  in  two  senses— the  subjective  (that  which  has  ref- 
erence to  our  mind)  and  the  objective  (that  which  refers  only  to  the  objects 
around  us).  (1)  Sound  is  the  sensation  produced  upon  the  organ  of  hearing 
by  vibrations  in  matter.  In  this  use  of  the  word  there  can  be  no  sound  where 
there  is  no  ear  to  catch  the  vibrations.  An  oak  falls  in  the  forest,  and  if  there 
is  no  ear  to  hear  it  there  is  no  noise,  and  the  old  tree  drops  quietly  to  its 
resting-place.  Niagara's  flood  poured  over  its  rocky  precipice  for  ages,  but 
fell  silently  to  the  ground.  There  were  the  vibrations  of  earth  and  air,  but 
there  was  no  ear  to  receive  them  and  translate  them  into  sound.  When, 
however,  the  first  foot  trod  those  primeval  solitudes,  and  the  ear  first  felt  the 
pulsations  from  the  torrent,  then  first  the  roaring  cataract  found  a  voice 
and  broke  its  lasting  silence.  A  trumpet  does  not  sound.  It  only  carves 
the  air  into  waves.  The  tympanum  is  the  beach  on  which  these  break  into 
sound.  (2)  Sound  is  those  vibrations  of  matter  capable  of  producing  a 
sensation  upon  the  organ  of  hearing.  In  this  use  of  the  word  there  can  be  a 
sound  in  the  absence  of  the  ear.  An  object  falls  and  the  vibrations  are  pro- 
duced, though  there  may  be  no  organ  of  hearing  to  receive  an  impression  from 
them. 


!  5  2  NA  TURAL  PHIL  OSOPHY. 

the  repeated  taps.  Place  your  cheek  near  them,  and 
you  will  feel  the  little  puffs  of  wind.  Insert  the 
handle  between  your  teeth,  and  you  will  experience 
the  indescribable  thrill  of  the  swinging  metal.  The 
tuning-fork  may  be  made  even  to  draw  the  outline 
of  its  vibrations  upon  a  smoked-glass.  Fasten  upon 
one  prong  a  sharp-pointed  piece  of  metal,  and  on 


Fig.  111. 


drawing  the  fork  along  as  in  the  figure,  a  sinuous 
line  will  show  the  width  (amplitude)  of  the  vibrations. 

From  many  similar  experiments  it  is  believed  that 
when  a  body  is  struck  its  molecules  are  thrown  into 
motion,  and  that  all  sound  is  produced  by  vibrations. 

How  sound  is  conveyed  through  the  air. — Let  us  im- 
agine the  prong  of  the  tuning-fork  used  in  the  last 
experiment  to  advance,  condensing  the  air  in  front 
of  it,  and  then  to  recede,  leaving  behind  it  a  partial 
vacuum.  This  process  is  repeated  until  the  fork 
comes  to  rest,  and  the  sound  ceases.  Each  vibration 
of  the  prong  produces  a  sound-wave  of  air,  which 
contains  one  condensation  and  one  rarefaction.  In 
water,  we  measure  a  wave-length  from  crest  to  crest ; 
in  air,  from  condensation  to  condensation.  The  con- 
densation of  the  sound-wave  corresponds  to  the  crest 


ACOUSTICS. 


of  the  water-wave,  and  the  rarefaction  of  the  sound- 
wave to  the  hollow  of  the  water-wave.    In  the  figure, 


the  dark  spaces  a,  b,  c,  d  represent  the  condensations, 
and  a,  &',  c  the  rarefactions ;  the  wave-lengths  are 
the  distances  ab,  &c,  cd. 

If  we  fire  a  gun,  the  gases  which  are  produced  ex- 
pand suddenly  and  force  the  air  outward  in  every 
direction.  This  hollow  shell  of  air  thus  condensed 
imparts  its  motion  to  that  next  to  it,  while  it  springs 
back  by  its  elasticity  and  becomes  rarefied.  The 
second  shell  rushes  forward  with  the  motion  re- 
ceived, then  bounds  back  and  becomes  rarefied. 
Thus  each  shell  of  air  takes  up  the  motion  and 
imparts  it  to  the  next.  The  wave,  consisting  of 
a  condensation  and  a  rarefaction,  proceeds  onward. 
It  is,  however,  as  in  water-waves,  a  movement  of  the 
form  only,  while  the  particles  vibrate  but  a  short 
distance  to  and  fro.  The  molecules  in  water-waves 
oscillate  vertically  /  those  in  sound-waves  horizontally , 
or  parallel  to  the  line  of  motion. 


154 


NATURAL  PHILOSOPHY. 


If  a  bell  be  rung,  the  air  adjacent  to  it  is  set  in 
motion  :  thence,  by  a  series  of  condensations  and 
rarefactions,  the  vibrations  of  the  bell  are  conveyed 
to  the  ear,  and  thus  produce  the  sensation  of  sound. 


Fig.  113. 


When  we  speak,  we  do  not  shoot  the  air  which  we 
expel  from  our  lungs  into  the  ear  of  the  listener.  We 
simply  condense  the  air  just  before  our  mouth  and 
throw  it  into  vibrations.  Thus  a  sound-wave  is 
formed.  This  travels  onward  and  spreads  in  every 
direction  in  the  form  of  a  sphere  of  which  we  are  the 
centre. 

SOUND  WILL  NOT  PASS  OUT  OF  A  VACUUM. — In 
the  figure,  B,  the  bell,  is  struck  by  clock-work  which 
may  be  set  in  motion  by  the  sliding-rod  r.  The  ap- 
paratus is  suspended  by  means  of  silk  cords,  that  no 
vibration  may  be  conducted  through  the  pump  itself. 
If  the  air  be  exhausted,  the  sound  will  become  so 
faint  that  it  cannot  be  heard,  even  when  the  ear  is 
placed  close  to  the  receiver. 


ACOUSTICS. 


155 


Fig.  114. 


There  is  perfect  silence  in  a  vacuum.  No  sound 
can  therefore  be  transmitted  to  the  earth  from  the 
regions  of  space.  The 
movements  of  the  heavenly 
bodies  are  noiseless.  In 
the  expressive  language  of 
David,  "  Their  voice  is  not 
heard."  In  elevated  re- 
gions sounds  are  dimin- 
ished in  loudness.  The 
explosion  of  a  pistol  on 
Mont  Blanc  is  said  to  re- 
semble that  of  an  ordinary 
fire-cracker ;  and  it  is  diffi- 
cult to  continue  a  conver- 
sation, as  the  voice  must 
be  raised  so  far  above  its 
natural  pitch.  The  reverse 
of  this  takes  place  when 
persons  descend  into  deep 
mines  or  in  diving-bells. 
The  sounds  then  become  startlingly  distinct,  and  the 
workmen  are  compelled  to  talk  in  whispers. 

THE  TELOCITY  or  SOUND  depends  on  the  elasticity 
and  density  of  the  medium  through  which  it  passes. 
The  higher  the  elasticity,  the  more  promptly  and  rap- 
idly the  motion  will  be  transmitted,  since  the  elastic 
force  acts  like  a  bent  spring  between  the  molecules  : 
the  greater  the  density,  the  more  molecules  to  be 
set  in  motion,  and  hence  the  slower  the  transmissi  n. 


156  NATURAL  PHILOSOPHY. 

Sound  travels  through  the  air  (at  tJie  freezing-point) 
at  the  rate  of  1,090  feet  per  second. — A  rise  in  temper- 
ature diminishes  the  density  of  the  air,  and  thus 
sound  travels  faster  in  warm  and  slower  in  cold  air. 
A  difference  of  1°  F.  makes  a  variation  of  about  1 
foot  in  velocity. 

Sound  travels  through  ivater  at  the  rate  of  4,700  feet 
per  second — Water  is  denser  than  the  air,  and  for  that 
reason  sound  should  travel  in  it  much  more  slowly ; 
but  its  elasticity,  which  is  measured  by  the  force  re- 
quired to  compress  it,  is  so  much  greater,  that  the 
rate  is  quadrupled. 

Sound  travels  through  solids  faster  than  through  air. 
— This  may  be  nicely  illustrated  by  placing  the  ear 
close  to  the  horizontal  bar  at  one  end  of  an  iron 
fence,  and  having  a  person  at  the  other  end  strike 
the  fence  a  smart  blow.  Two  successive  sounds  will 
reach  the  ear — one  through  the  metal,  and  afterward 
another  through,  the  air.  In  some  experiments  made 
by  Biot,  when  a  bell  was  struck  at  the  end  of  an  iron 
tube  3,120  feet  long,  2|  seconds  elapsed  between  the 
two  sounds.  This  would  make  the  velocity  in  iron 
nearly  ten  times  that  in  air. 

Sounds  travel  ivith  the  same  velocity. — Under  ordi- 
nary circumstances  we  see  that  this  must  be  true.  A 
band  is  playing  at  a  distance,  yet  the  harmony  of  the 
different  instruments  is  preserved.  The  soft  and  loud, 
high  and  low  notes  reach  the  ear  at  the  same  time. 
It  has  been  said  that  the  "  heaviest  thunder  trav- 
els no  faster  than  the  softest  whisper."  This  is  not 


ACOUSTICS.  j^y 

verified  by  careful  investigations.  Mr.  Mallet  found 
that  in  blasting  with  a  charge  of  2,000  Ibs.,  the  ve- 
locity was  967  feet  per  second,  while  with  a  charge  of 
12,000  Ibs.  it  was  increased  to  1,210  feet  per  second. 
Capt.  Parry  in  his  Arctic  travels  made  a  similar 
observation.  He  states  that  on  a  certain  occasion 
when  at  a  considerable  distance,  the  sound  of  the 
sunset-gun  reached  his  ears  before  the  officer's  word 
of  command  to  fire,  proving  that  the  report  of  the 
cannon  travelled  sensibly  faster  than  the  sound  of 
the  voice. 

The  velocity  of  sound  may  be  used  to -determine  dis- 
tances.— Light  travels  instantaneously  as  far  as  all 
distances  on  the  earth  are  concerned.  Sound  moves 
more  slowly.  For  this  reason  we  see  a  chopper 
strike  with  his  axe,  and  a  moment  elapses  before  we 
hear  the  blow.  If  the  time  that  intervenes  is  one 
second,  we  know  that  the  distance  is  about  1,090  feet. 
By  means  of  the  second-hand  of  a  watch  or  the 
beating  of  our  pulse,  we  can  count  the  seconds  that 
elapse  between  a  flash  of  lightning  and  the  peal 
of  thunder  which  follows.  Multiplying  the  velocity 
of  sound  by  the  number  of  seconds,  we  have  the  dis- 
tance of  the  thunderbolt. 

THE  INTENSITY  OF  SOUND  depends  upon  the  ampli~ 
tude  of  the  vibrations. — The  amplitude  is  the  dis- 
tance the  molecules  swing  to  and  fro.  As  in  a 
pendulum,  the  greater  the  amplitude,  the  greater 
the  velocity.  In  momentum,  we  found  that  the  force 
of  a  striking  body  depends  upon  its  weight  and  its 


i58 


NA  TURAL  PHIL  OSOPHY. 


velocity.  Just  so,  if  one  sound  appears  to  us  louder 
than  another,  it  is  because  the  air  molecules  hit  the 
ear-drum  with  greater  force.  On  the  top  of  a  moun- 
tain, because  of  the  rareness  of  the  atmosphere,  there 
are  fewer  molecules  to  strike  the  ear;  hence,  ac- 
cording to  the  principles  of  momentum,  the  blow 
will  be  less  intense.  i 

The  intensity  of  sound  diminishes  as  the  square  of  the 
distance  increases.* — This  is  the  natural  effect  of  the 
expanding  of  the  sound-wave,  which  proceeds  in  the 
form  of  a  sphere.  The  larger  the  sphere,  the  greater 
the  number  of  air  particles  to  be  set  in  motion,  and 
hence  the  feebler  their  vibration.  The  surfaces  of 
spheres  are  proportional  to  the  squares  of  their  radii. 
The  radii  of  sound-spheres  are  their  distances  from 
the  centre  of  disturbance.  Hence  the  force  with 
which  the  molecules  will  strike  our  ear  decreases  as 
the  square  of  our  distance  from  the  sounding  body. 

Speaking-tubes  conduct  sound  to  distant  rooms  be- 
cause they  prevent  the  waves  from  expanding  and 
losing  their  intensity.  *  Biot  found  that  a  conversa- 
tion in  a  low  tone  could  be  kept  up  through  a  Paris 
water-pipe  3,120  feet  long.  He  says  that  "it  was  so 

*  The  same  proportion  obtains  in  Gravitation,  Sound,  Light, 
and  Heat.  We  have  seen  how  the  pendulum  is  based  upon  the 
force  of  gravity,  and  reveals  the  laws  of  falling  bodies.  Now  we 
find  that  ihe  pendulum,  and  even  the  principles  of  reflected  mo- 
tion and  momentum,  are  linked  with  the  phenomena  of  sound 
As  we  progress  further,  we  shall  find  how  Nature  is  thus  inter- 
woven everywhere  with  proofs  of  a  common  plan  and  a  common 
Author. 


ACOUSTICS.  1^g 

easy  to  be  heard,  that  the  only  way  not  to  be  heard 
was  not  to  speak  at  all."  A  communication  could  be 
made  in  this  manner  even  between  two  villages.  The 
ear-trumpet  acts  by  collecting  waves  of  sound  and  re- 
flecting them  into  the  ear.  The  speaking-trumpet  is 
often  explained  on  the  same  principle  as  the  speak- 
ing-tube. A  more  rational  theory  is,  that  the  sound 
of  the  voice  is  strengthened  by  the  vibrations  of  the 
air  in  the  tube. 

EEFRACTION  OF  SOUND. — "When  a  sound-wave  goes 
obliquely  from  one  medium  to  another,  it  is  bent  out 
of  its  direct  course.  It  may  even,  like  light,  be 
passed  through  a  lens  and  brought  to  a  focus.  B  is 
a  thin  rubber  balloon,  filled  with  carbonic  acid  gas  ; 
10  is  a  watch,  and  /'  is  a  glass  funnel  which  as- 

Fig.  115. 


sists  in  collecting  the  wave  at  /,  where  the  ear  is 
placed.  By  moving  the  head,  a  point  will  soon 
be  found  where  the  ticks  of  the  watch  can  be  heard 
very  distinctly,  while  outside  of  it  they  are  inau- 
dible. 


j  60  NA  TURAL  PHIL  OSOPHY. 

REFLECTION  OF  SOUND. — When  a  sound-wave  strikes 
against  the  surface  of  another  medium,  a  portion 
goes  on  while  the  rest  is  reflected. 

The  law  which  governs  Reflected  Sound  is  that  of 
Reflected  Motion  ; — the  angle  of  incidence  is  equal  to 
that  of  reflection.  Tyndall  relates  that  a  bell  on  a 
distant  eminence  in  Heligoland  failed  to  be  heard 
in  the  town.  A  reflector  was  therefore  placed  behind 
it,  so  as  to  throw  the  sound-waves  in  the  direction  of 
the  long  sloping  street.  This  caused  every  stroke 
to  be  distinctly  audible.  Domes  and  curved  walls 
and  ceilings  act  in  the  same  manner  as  mirrors  in 
the  reflection  of  sound.  Sir  John  Herschel  relates 
an  amusing  illustration  of  this  fact.  A  confes- 
sional in  a  cathedral  in  Sicily  was  so  situated  that 
the  whispers  of  the  penitents  were  reflected  by  the 
curved  roof  and  brought  to  a  focus  at  a  distant  part 
of  the  edifice.  This  point  was  accidentally  discov- 
ered by  a  gentleman,  who  amused  himself  and  his 
friends  by  listening  to  utterances  intended  for  the 
ear  of  the  priest  alone.  One  day,  however,  his  wife 
was  the  penitent,  and  both  he  and  his  friends  were 
thus  made  acquainted  with  family  secrets  which 
were  as'  new  to  himself  as  they  were  the  reverse  of 
amusing.  "  The  Ear  of  Dionysius"  was  a  dungeon 
fn  Syracuse,  so  constructed  as  to  convey  to  the  ears 
of  that  tyrant  every  word  spoken  by  its  unfortunate 
inmates.  "Whispering  galleries  are  commonly  made 
of  an  elliptical  form.  Two  persons,  standing  at  the 
foci  with  their  backs  to  each  other,  can  thus  carry  on 


ACOUSTICS.  l()l 

a  conversation  in  whispers  which  are  entirely  un- 
noticed by  those  between  them.  Sound-waves  have 
been  brought  to  a  focus  by  the  mainsail  of  a  vessel 
having  accidentally  taken  a  concave  form  ;  in  this 
manner  a  bell  was  once  heard  100  miles  out  at  sea. 

Decrease  of  Sound  by  Reflection. — If  we  strike  the 
bell  represented  in  Fig.  114,  we  find  a  great  differ- 
ence between  its  sound  under  the  glass  receiver  and 
in  the  open  air.  Floors  are  deadened  with  tan-bark 
or  other  fine  material ;  since,  as  the  sound-wave  passes 
from  each  particle  to  the  next  of  the  unhomogene- 
ous  mass,  it  becomes  weakened  by  partial  reflection. 
During  a  thunder-storm  the  air  is  of  such  varying 
density  that  thunder-peals  are  never  heard  at  a  dis- 
tance corresponding  to  their  violence.  For  the  same 
reason,  the  roar  of  cannon  on  a  field  of  battle  is  not 
noticeable,  and  the  day  has  often  been  lost  within 
H  short  distance  of  the  reserves  of  the  defeated 
army,  which  were  waiting  for  the  sound  of  artillery 
to  call  them  to  the  scene  of  action.  The  air  at 
night  is  more  homogeneous,  and  hence  sounds  are 
heard  more  clearly  and  farther  than  in  the  -daytime. 
In  foggy  weather  sounds  suffer  innumerable  reflec- 
tions from  the  mist,  and  are  soon  destroyed. 
(Tyndatt.) 

Resonance. — If  the  reflecting  surface  be  very  near, 
the  reflected  sound  will  join  the  direct  one  and 
strengthen  it.  This  effect  is  termed  a  resonance.  It 
accounts  for  the  well-known  fact  that  a  speaker  can 
be  heard  much  more  easily  in  a  close  room  than  in 


!  6  2  NA  TURAL  PHIL  OSOPHY. 

the  open  air.  A  smooth  wall  back  of  the  stand 
re-enforces  the  voice  in  the  same  manner.  The  old- 
fashioned  sounding-boards  were  by  no  means  ineffi- 
cient, however  inelegant  may  have  been  their  appear- 
ance. Shells,  by  their  peculiar  convolutions,  reflect 
and  augment  the  various  sounds  which  fill  even  the 
stillest  air.  As  we  hold  them  to  our  ear,  they  are 
poetically  said  to  "  repeat  the  murmurs  of  their 
ocean-home."  Furniture  and  wall-hangings  break 
up  the  resonance  of  a  room  ;  and  thus  our  footsteps 
in  unfurnished  dwellings  sound  startlingly  distinct. 
Echoes  are  produced  where  the  reflecting  surface  is 
so  distant  that  we  can  distinguish  the  reflected 
sound  from  the  direct  one.  If  the  sound  be  short 
and  quick,  this  requires  at  least  56  feet ;  but  if  it  be 
an  articulate  one,  112  feet  are  necessary.  No  one 
can  pronounce  or  hear  distinctly  more  than  five  syl- 
lables in  a  second;  1,120  ft. -f- 5=  224  ft.*  If  the  wave 
travel  224  feet  in  going  and  returning,  the  two 
sounds  will  not  blend,  and  the  ear  can  detect  a  dis- 
tinct interval  between  them.  A  person  speaking  in 
a  loud  voice  in  front  of  a  mirror,  112  feet  distant, 
can  distinguish  the  echo  of  the  last  syllable  he  utters ; 
if  twice  that,  or  224  feet,  the  last  two  syllables,  etc. 
Places  where  good  echoes  may  be  heard  abound  in 
every  locality.  When  several  parallel  surfaces  are 
properly  situated,  the  echo  may  be  repeated  back- 

*  This  calculation  supposes  the  sound  to  travel  at  the  rate  of 
1,120  feet  per  second. 


ACOUSTICS. 


ward  and  forward  in  a  surprising  manner.  At 
Woodstock,  England,  is  one  which  repeats  17  sylla- 
bles by  day  and  20  by  night.  The  reflecting  surface 
is  distant  about  2,300  feet;  a  quick, sharp  ha!  will 
come  back  a  ringing  ha,  ha,  ha  !  The  echo  is  often 
softened,  as  in  the  Alpine  regions,  where  it  warbles  a 
beautiful  accompaniment  to  the  shepherd's  horn. 

THE   DIFFERENCE  BETWEEN  NOISE  AND  MUSIC  is    Only 

that  between  irregular  and  regular  vibrations.  What- 
ever may  be  the  cause  whicli  sets  the  air  in  motion, 
if  the  vibrations  be  uniform  and  rapid  enough,  the 
sound  is  musical.  If  the  ticks  of  a  watch  could  be 
made  with  sufficient  rapidity,  they  would  lose  their 
individuality  and  blend  into  a  musical  tone.  "  The 
puffs  of  a  locomotive  are  slow  on  first  starting,  but 
they  soon  increase  so  as  to  be  almost  incapable  of 
being  counted.  If  the  puffs  could  reach  50  or  60  a 
second,  the  approach  of  an  engine  would  be  her- 
alded by  an  organ-peal  of  tremendous  power." 

Nothing  can  be  imagined  to  be  more  purely  a 
noise  than  the  rattling  of  a  cab  over  a  stony  street. 
The  pavement  of  London  is  composed  of  granite 
blocks,  four  inches  in  width.  A  cab-wheel  jolting 
over  this  at  the  rate  of  eight  miles  per  hour  produces 
a  succession  of  35  distinct  sounds  per  second.  These 
link  themselves  together  into  a  soft,  deep  musical 
tone,  that  will  bear  comparison  with  notes  derived 
from  more  sentimental  sources.  (Houghton.) 

PITCH. — If  we  hold  a  card  against  the  cogs  of 
the  wheel  in  the  apparatus  shown  in  Fig.  32,  when 


1 64 


NATURAL  PHILOSOPHY. 


turned  rapidly  we  shall  obtain  a  pure,  clear  tone ;  and 
the  faster  the  wheel  is  revolved,  the  shriller  the  tone, 
or  the  higher  the  pitch.  Hence  we  conclude  that 

Pitch  depends  on  the  ra- 
pidity of  the  vibrations. 

HOW  TO  FIND  THE  NUM- 
BER OF  WAVES  IN  A  MUSI- 
CAL SOUND. — This  is  de- 
termined by  means  of  an 
instrument  called  the  Si- 
ren. C  is  a  cylindrical 
box  ;  t,  a  pipe  for  admit- 
ting the  air  ;  a  &,  a  plate 
pierced  with  four  series 
of  holes,  containing  8, 
10,  12,  and  16  orifices 
respectively ;  m,  n,  o,  p 
are  stops  for  closing  any 
series  at  pleasure.  The 
vertical  rod  p  is  bevelled 
at  p  so  as  to  turn  in  the 
socket  x ;  d  e  is  a  disk 
pierced  with  holes  cor- 
responding to  those  in 
the  lower  plate,  over 
which  it  is  made  to  re= 
volve.  At  s  is  an  end- 
less screw,  which,  as  the 
axis  p  revolves,  causes  two  wheels  to  rotate,  and 
thus  turns  the  hands  upon  the  dial  (Fig.  118).  On 


ACOUSTICS. 


i65 


Fig.  117. 


this  we  can  see  at  any  moment  the  number  of 
revolutions  made  by  the  upper  disk.  The  holes 
in  a  b  and  d  e  are  inclined  to  each  other,  so  that, 
when  a  current  of 
air  is  forced  in  at  t, 
it  passes  up  through 
the  openings  in  the 
lower  disk,  and  strik- 
ing against  the  sides 
of  those  in  the  upper 
disk,  causes  it  to  re- 
volve. As  the  upper 
disk  turns  round,  it 
alternately  opens  and 
closes  the  orifices  in 
the  lower  disk,  and 
thus  converts  the 
steady  stream  of  air 
into  uniform  puffs. 
At  first  they  succeed 
each  other  so  slowly 
that  they  may  be 
easily  counted.  At 
last,  as  the  motion 
increases,  they  link 
themselves  together, 
and  burst  into  a  full, 
melodious  note.  As  the  velocity  augments,  the  pitch 
rises,  until  the  music  becomes  so  shrill  as  to  be  painful. 
Diminish  the  speed,  and  the  pitch  falls  immediately. 


1 66 


NATURAL  PHILOSOPHY. 


Let  us  now  see  how  the  Siren  is  used  to  determine 
the  number  of  vibrations  in  any  sound.     Force  the 
pig  118  air  through  it  stead- 

ily until  the  tone  is 
brought  to  any  re- 
quired pitch.  Find 
on  the  dial,  at  the 
|a  end  of  a  minute,  the 
number  of  revolu- 
tions made  by  the 
disk.  When  the  row 
containing  ten  holes 
is  open,  and  the  tone 
C2,  it  will  indicate  1,536.  There  were  ten  puffs 
of  air,  or  ten  waves  of  sound,  in  each  revolution. 
1,536  X  10  =  15,360.  Dividing  this  by  60,  we  have 
256  as  the  number  per  second.  When  the  inner 
and  outer  rows  of  holes  are  opened,  the  ear  im- 
mediately detects  the  difference  of  an  octave  be- 
tween the  two  sounds.  The  one  containing  8  pro- 
duces the'  lower,  and  the  one  containing  16  the 
higher  tone.  Hence  we  conclude  that  an  octave 
of  any  tone  is  caused  by  double  the  number  of 
vibrations. 

How  to  find  the  length  of  the  tvave  in  a  musical 
sound. — Suppose  the  air  in  the  last  experiment  was 
of  such  a  temperature  that  the  foremost  sound-wave 
would  have  reached  the  distance  of  1,120  feet  in  a 
second.  In  that  space  there  were  256  sound-waves. 
Dividing  1,120  by  256,  we  have  4  feet  4  inches  as  the 


ACOUSTICS. 


I67 


length  of  each.  We  see  from  this  that  we  find  the 
wave-length  by  dividing  the  velocity  of  sound  by 
the  number  of  vibrations  per  second.  As  the  pitch 
is  elevated  by  rapidity  of  vibration,  we  readily  per- 
ceive that  the  low  tones  in  music  are  produced  by 
the  long  waves  and  the  high  tones  by  the  short 
waves.  An  experiment  illustrative  of  this  can  be 
made  when*  an  express-train  passes  a  railroad  sta- 
tion. As  the  engine  approaches  us,  the  waves  from 
the  whistle  are  shortened  by  the  rapid  motion,  and 
as  it  recedes,  are  lengthened ;  the  pitch  of  the 
whistle  will  therefore  be  raised  as  the  train  comes 
in,  and  be  lowered  as  it  goes  out.  The  same  result 
may  be  detected  if  a  person  in  a  high  swing  pro- 
duces, while  in  swift  motion,  a  continuous  musical 
tone  upon  some  instrument. 

Application  to  any  Musical  Sound. — Whenever  notes 
from  any  two  sources  are  in  unison,  they  are  pro- 
duced by  the  same  number  of  vibrations.  If  the 
string  of  a  violin,  the  cord  of  a  guitar,  the  parch- 
ment of  a  drum,  and  the  pipe  of  an  organ,  produce 
the  same  musical  tone,  it  is  because  the  vibrations 
in  all  are  isochronous.  "  If  a  voice  and  a  piano 
execute  the  same  music,  the  steel  strings  of  the 
piano  and  the  vocal  cords  of  the  singer  vibrate 
together  and  send  out  sound-waves  of  the  same 
length."  In  order,  then,  to  determine  the  number 
and  length  of  the  sound-waves  produced  by  a  sono- 
rous body,  we  have  only  to  bring  its  sound  and  that 
of  the  siren  in  unison.  In  this  way  it  has  been  found 


1 6  8  NA  TURAL  PHIL  OSOPHY, 

that  the  wings  of  a  gnat  flap,  in  flying,  at  the  rate 
of  15,000  times  per  second.  The  waves  produced 
by  a  man's  voice  in  ordinary  conversation  are  from 
eight  to  twelve  feet  in  length,  and  by  a  woman's 
voice  from  two  to  four  feet.  (Tyndall.) 

SUPER-POSITION  OF  SOUND-WAVES. — (See  "Wave  Mo- 
tion, p.  128.) — The  air  may  transmit  sound- waves  from 
a  thousand  instruments  at  once.  If  tKe  condensa- 
tion of  one  wave  meet  the  condensation  of  another, 
it  will  augment  the  sound,  the  condensations  becom- 
ing more  condensed  and  the  rarefactions  more  rare- 
fied by  their  coincidence.  If,  on  the  other  hand,  the 
condensation  of  one  meet  the  rarefaction  of  the 
other,  the  result  will  be  changed  ;  one  wave  motion 
will  be  striving  to  push  the  air  molecules  forward, 
and  the  other  to  urge  them  backward.  Thus,  if 
they  meet  in  exactly  opposite  phases  and  the  two 
forces  are  equal,  they  will  balance  each  other  and 
silence  will  ensue.  Thus  a  sound  added  to  a  sound 
may  produce  silence.  In  the  same  way,  two  motions 
may  produce  rest ;  two  lights  may  cause  darkness  ;  and 
two  heats  may  produce  cold. 

Fig.  119. 


Suppose   we    have    two    tuning-forks,  A  and  B, 
placed  a  wave-length  apart,  and  vibrating  in  unison. 


ACOUSTICS. 


169 


The  waves  from  the  two  will  coincide,  as  represented 
by  the  light  and  dark  shades  in  the  figure.  The 
same  would  occur  if  they  were  placed  at  any  num- 
ber of  wave-lengths  apart.  If  they  are  a  half  wave- 
length apart,  the  condensation  of  A  coincides  with 
the  rarefaction  of  B,  and  vice  versa.  The  effect  is 
represented  by  the  uniformity  of  the  shading  in  Fig. 
120.  This  is  termed  interference  of  sound-waves. 

Fig.  120. 


There  are  positions  in  which  the  prongs  of  a 
tuning-fork  interfere  with  each  other  so  as  to  pro- 
duce silence.  If  we  strike  the  fork  and  turn  it 
slowly  around  before  the  ear,  we  shall  find  four  points 
where  the  interference  of  the  sound-waves  entirely 
neutralizes  the  vibrations. 

VIBEATIONS  OF  CORDS. — Let   a  b  be   a   stretched 


cord  made  to  vibrate.  The  motion  from  e  to  d  and 
back  again  is  termed  a  complete  vibration;  that 
from  e  to  d  alone,  is  a  half- vibration.  The  intensity 
of  the  sound  depends  on  the  width  of  e  d--  i.  e.,  the 


170 


NATURAL  PHILOSOPHY 


amplitude  of  the  vibration.  It  is,  however,  very 
weak,  on  account  of  the  small  amount  of  air  a  sim- 
ple cord  can  set  in  motion.  The  laws  which  govern 
the  number  of  vibrations,  and  hence  the  pitch,  are 
investigated  by  means  of  an  instrument  known  as 
the  SONOMETEB.  It  consists  of  two  cords  stretched 
by  weights  at  P,  across  two  fixed  bridges,  A  and  B. 


Fig.  122. 


D  is  a  movable  bridge,  which  serves  to  lengthen  or 
shorten  the  cords  at  pleasure.  Beneath  is  a  wooden 
box  which  receives  the  vibrations  of  the  cords  and 
communicates  them  to  the  air  within.  This  is  the 
real  sounding  body. 

1st  Law.  The  number  of  vibrations  per  second  in- 
creases as  the  length  of  the  cord  decreases. — Let  the 
cord  be  caused  to  vibrate,  and  we  shall  hear  the  note 
of  the  entire  string.  Now  place  the  movable  bridge 
D  at  the  centre  of  the  cord,  and  we  shall  obtain  a 
sound  the  octave  of  the  former.  Thus  by  taking 


ACOUSTICS. 


171 


one-half  the  length  of  the  cord  we  double  the  num- 
ber of  vibrations.  If  an  entire  cord  makes  20 
vibrations  per  second,  one-half  will  make  40,  and 
one-third,  60.  The  violin  or  guitar  player  elevates 
the  pitch  of  any  string  of  his  instrument  by  moving 
his  finger,  and  thus  shortening  the  length  of  the 
vibrating  portion.  In  the  piano,  harp,  etc.,  the 
long  and  short  strings  produce  the  low  and  high 
notes  respectively. 

2d  Law.  The  number  of  vibrations  per  second 
increases  as  the  square-root  of  the  tension. — The  cord 
when  stretched  by  1  Ib.  gives  a  certain  tone  :  to 
double  the  number  of  vibrations  and  obtain  the 
octave  requires  a  weight  of  4  Ibs.  All  stringed  in- 
struments are  provided  with  keys,  by  means  of  which 
the  tension  of  the  cord  and  the  corresponding  pitch 
may  be  increased  or  diminished. 

3d  Law.  The  number  of  vibrations  per  second  de- 
creases as  the  square-root  of  the  weight  of  the  cord  in- 
creases.— If  two  strings  of  the  same  material  be 
equally  stretched,  and  one  have  four  times  the  weight 
of  the  other,  it  will  only  vibrate  half  as  often.  In 
the  violin  the  bass  notes  are  produced  by  the  thick 
strings.  In  the  piano  the  result  is  obtained  by  coil- 
ing fine  wire  around  the  heavy  strings. 

NODES. — In  the  experiments  just  named,  the  cord 
was  shortened  by  means  of  a  movable  bridge  which 
held  it  firmly  at  the  centre.  If,  instead,  we  simply 
rest  the  feather-end  of  a  goose-quill  lightly  on  the 
string,  and  then  draw  the  bow  over  one-half,  it  will 


172 


NATURAL  PHILOSOPHY 


vibrate  in  two  portions  and  will  give  the  octave  as 
before.     Remove  the  feather,  and  it  will  continue  to 


Fig.  123. 


vibrate  in  two  parts  and  to  yield  the  same  tone. 
We  can  show  that  the  second  half  vibrates  by  sim- 
ply placing  across  the  middle  of  that  portion  of  the 
wire  a  little  paper  rider.  On  drawing  the  bow  the 


Fig.  124. 


rider  will  be  thrown  off.  Hold  the  feather  so  as  to 
separate  one-third  of  the  string  and  cause  it  to 
vibrate.  The  remainder  of  the  cord  will  vibrate 
in  two  segments.  When  the  feather  is  removed,  the 


ACOUSTICS. 


173 


entire  cord  will  vibrate  in  three  different  parts  of 
equal  length,  separated  by  stationary  points  called 
nodes.  This  may  be  shown  by  placing  on  the  wire 
three  riders ;  the  one  at  the  node  will  remain,  while 
the  others  will  be  thrown  off.  In  the  same  manner 
the  cord  may  be  divided  into  any  number  of  equal 
vibrating  segments  and  stationary  nodes. 

Acoustic  Figures. — Sprinkle  some  fine  sand  on  a 
glass  or  metal  plate.     Place  the  finger-nail  on  one 

Fig.  125. 


edge  to  stop  the  vibration  at  that  point,  as  the 
feather  did  in  the  last  experiment,  and  draw  the  bow 
lightly  across  the  opposite  edge.  The  sand  will  be 
tossed  away  from  the  various  parts  of  the  plate  and 
will  collect  along  two  nodal  lines,  which  divide  the 
large  square.  It  is  wonderful  to  see  how  the  sand 
will  seemingly  start  into  life  and  dance  into  line  at 
the  touch  of  the  bow.  Fig.  126  shows  some  of  the 
beautiful  patterns  obtained  by  Chladni. 


174 


NATURAL  PHILOSOPHY. 
Fig.  126. 


ACOUSTICS. 


175 


Harmonics  or  overtones. — Even  when  a  cord  is  caused 
to  vibrate  in  its  full  length,  it  separates  into  parts  at 
the  same  time.  Thus  we  have  the  full  or  fundamen- 
tal note  of  the  entire  string ;  and  superposed  upon 
that,  the  higher  notes  produced  by  the  vibrating 
parts.  These  are  called  overtones  or  harmonics. 
The  overtones  vary  in  different  instruments.  The 
mingling  of  the  two  classes  of  vibrations  determines 
the  quality  of  the  sound,  and  enables  us  to  distin- 
guish the  music  of  different  instruments. 

Nodes  of  a  Bell. — Let  the  heavy  circle  in  Fig.  127, 
represent  the  circumference  of  a  bell  when  at  rest. 
Let  the  hammer  strike  at 
a,  b,  c,  or  d.  At  one  moment 
as  the  bell  vibrates  it  will 
form  an  oval  with  a  Z>,  at 
the  next  with  c  d  for  its 
longest  diameter.  When 
it  strikes  its  deepest  note, 
the  bell  vibrates  in  four 
segments,  with  n,  n,  n,  n,  as 
the  nodal  points,  whence 
nodal  lines  run  up  from  the  edge  to  the  crown  of  the 
bell.  It  tends,  however,  to  divide  into  a  greater 
number  of  segments,  especially  if  it  is  very  thin, 
and  so  to  produce  a  series  of  harmonic  sounds. 
These  overtones,  which  follow  the  deep  tones  of  the 
bell,  are  frequently  very  striking,  even  in  a  common 
call-bell. 

Nodes  of  a  Sounding-board. — The  case  of  the  violin 


jyg  NATURAL  PHILOSOPHY. 

or  guitar  is  composed  of  thin  wooden  plates  which 
divide  into  vibrating  segments,  separated  by  nodal 
lines  according  to  the  pitch  of  the  note  which  is 
being  played.  The  enclosed  air  vibrating  in  unison 
with  these,  re-enforces  the  sound  and  gives  it  fulness 
and  richness.  The  sounding-board  of  the  piano 
acts  in  a  similar  manner. 

THE  MUSICAL  SCALE. — The  tone  produced  by  the 
vibrations  of  an  entire  string  is  called  its  fundamen- 
tal sound.  The  various  sounds  of  the  scale  above 
this  are  given  by  the  parts  of  the  string  indicated  by 
the  fractions 

C,        D,        E,        F,        G,        A,        B,        C. 

1        %        V5        3A      Vs        3/5      Vis        Va 

As  the  number  of  vibrations  varies  inversely  as 
the  length  of  the  cord,  we  need  but  to  invert  these 
fractions  to  obtain  the  relative  number  of  vibrations 
per  second  ;  thus,  -|,  f, -J,  I,  f ,  V>  2.  Reduced  to  a 
common  denominator,  their  numerators  are  propor- 
tional, and  we  have  the  whole-numbers  which  repre- 
sent the  relative  rates  of  vibration  of  the  notes  of 
the  scale,  viz. : 

24,    27,    30,    32,    36,    40,    45,    48. 

The  number  of  vibrations  corresponding  to  the 
different  letters  is, 

C,  D,  E,  F,  G,         A,  B,  C. 

128,          144,          160,          170.          192,        214,          240,         256. 

WIND  INSTRUMENTS  produce  musical  sounds  by 
means  of  enclosed  columns  of  air.  Sound-waves 
run  backward  and  forward  through  the  tube  and  act 
on  the  surrounding  air  like  the  vibrations  of  a  cord. 
The  sound-waves  in  organ-pipes  are  set  in  motion 


ACOUSTICS. 


177 


either  by  means  of  fixed  mouth-pieces  or  vibrating 
reeds.     The  air  is  forced  from  the  bellows  into  the 

Fig.  128. 


tube  P,  through  the  vent  i,  and  striking  against  the 
thin  edge  a,  produces  a  flutter.  The  column  of  air 
above,  being  thus  thrown  into  vibration,  re-enforces 
the  sound  and  gives  a  full  musical  tone.  The 
length  of  the  pipe  determines  the  pitch.  The  varia- 
tion in  the  quality  of  different  wind-instruments  is 
caused  by  the  mingling  of  the  harmonics  with  the 
fundamental  tone.  In  the  flute,  for  example,  the 


1 78 


NATURAL  PHILOSOPHY. 


vibrating  column  of  air  may  be  made  to  break  up 
into  vibrating  segments  with  stationary  nodes,  by 
merely  varying  the  force  of  the  breath. 

SYMPATHETIC  VIBBATTONS. — Stand  near  a  piano 
and  produce  a  musical  tone  with  the  voice,  and  you 
will  find  that  a  certain  wire  selects  that  pulse  of  sound 
and  responds  to  it.  Change  the  pitch,  and  the  first 
string  ceases,  while  another  replies.  If  a  hundred 
tuning-forks  of  different  tones  be  made  to  sound  at 
the  foot  of  an  organ-pipe,  it  will  choose  the  one  to 
which  it  is  able  to  reply,  and  respond  to  that  alone. 
Two  clocks  set  on  one  shelf  or  against  the  same 
wall,  affect  each  other.  "Watches  in  the  shop-win- 
dow keep  better  time  than  when  carried  singly. 

THE  EAE. — Fig.  129  is  a  sketch  of  the  ear,  drawn 

from  a  model.  The 

Fig.  129. 

small  bones  are 
much  magnified,  in 
order  to  give  a 
distinct  idea  of 
their  shape.  A  F 
represents  a  rear 
view  of  the  outer 
ear.  The  sound- 
wave passes  into 
the  auditory  canal, 
which  is  about  one 
inch  in  length,  and 
striking  against  the 
tympanum  or  drum,  E,  which  closes  the  orifice 


ACOUSTICS.  ^ 

of  the  external  ear,  throws  this  membrane  into 
vibration.  Next,  the  series  of  small  bones,  a,  b,  c, 
called  respectively,  from  their  peculiar  form,  the 
hammer,  anvil,  and  stirrup,  conduct  it  to  the  inner 
ear,  which  is  termed,  from  its  complicated  structure, 
the  labyrinth.  This  is  filled  with  liquid,  and  contains 
the  semi-circular  canals,  B,  and  the  cochlea  (snail- 
shell),  C,  which  receive  the  vibrations  and  transmit 
them  to  the  auditory  nerve,  the  fine  filaments  of  which 
are  spread  out  to  catch  every  pulsation  of  the  sound- 
wave. The  middle  ear,  which  contains  the  chain  of 
small  bones,  is  a  simple  cavity  about  J  inch  in  diam- 
eter, filled  with  air.  It  communicates  with  the 
mouth  by  means  of  the  Eustachian  tube,  D.  Within 
the  labyrinth  are  also  fine,  elastic  hair-bristles  and 
crystalline  particles  among  the  nerve-fibres,  wonder- 
fully fitted,  the  one  to  receive  and  the  other  to  pro- 
long the  vibrations ;  and  lastly,  a  lute  of  3,000  micro- 
scopic strings,  so  stretched  as  to  vibrate  in  unison 
with  any  sound.  The  Eustachian  tube  is  generally 
closed,  thus  cutting  off  the  air  in  the  inner  cavity 
from  the  external  air.  If  at  any  time  the  pressure 
of  the  atmosphere  without  becomes  greater  or  less 
than  that  within,  the  tympanum  feels  the  strain, 
pain  is  experienced,  and  partial  deafness  ensues. 
A  forcible  concussion  frequently  produces  in  this 
way  a  temporary  deafness.  In  the  act  of  swallow- 
ing, the  tube  is  opened  and  the  equilibrium  restored. 
We  may  force  air  into  the  cavity  of  the  ear  by 
closing  our  mouth  and  nose,  and  forcibly  expiring 


I  go  NA  TURAL  PHIL  OSOPHY. 

the  air  from  our  lungs.  This  will  render  us  insensi- 
ble to  low  sounds,  as  the  rumble  of  a  railway-train, 
while  we  can  hear  the  higher  ones  as  usual. 

LIMITS  OF  HEARING.— Helmholtz  fixes  the  lowest 
limit  of  musical  sounds  at  16  vibrations  per  second,* 
and  the  highest  at  38,000.  Below  this  number  the 
'pulses  cease  to  link  themselves  together,  and  be- 
come distinct  sounds.  The  range  of  the  ear  is  thus 
about  eleven  octaves.  The  practical  range  of  music 
is,  however,  only  about  seven  octaves.  The  capacity 
to  hear  the  higher  tones  varies  in  different  persons. 
A  sound  which  is  entirely  audible  to  one  may  be 
utter  silence  to  another.  Some  ears  cannot  distin- 
guish the  squeak  of  a  bat  or  the  chirp  of  a  cricket, 
while  others  are  acutely  sensitive  to  these  shrill 
sounds.  Indeed,  the  auditory  nerve  seems  generally 
more  alive  to  the  short  quick  vibrations  than  to  the 
long  slow  ones.  The  whirr  of  a  locust  is  much  more 
noticeable  than  the  sighing  of  the  wind  through  the 
trees.  To  this,  however,  there  are  noticeable  excep- 
tions. The  author  knows  of  a  person  who  is  en- 
tirely insensible  to  the  higher  tones  of  the  voice,  but 
acutely  sensitive  to  all  the  lower  ones.  Thus  on 
one  occasion,  being  in  a  distant  room,  she  did  not 
notice  the  ringing  of  the  bell  announcing  dinner, 
but  heard  the  noise  the  bell  made  when  returned 

*  A  tone  produced  by  about  32  vibrations  per  second  may  be 
made  by  inserting  the  finger  lightly  into  the  ear,  bringing  at  the 
same  time  the  muscles  of  the  hand  into  strong  contraction.  A 
sound  will  be  heard  which  is  as  deep  as  the  toll  of  a  cathedral  bell 


ACOUSTICS.  jgi 

to  its  place  on  the  shelf.  A  continuous  blast  of  air 
has  no  effect  to  produce  sound.  The  rush  of  the 
grand  aerial  rivers  above  us  we  never  hear.  They 
flow  on  ceaselessly  but  silently  in  the  upper  regions 
of  the  air,  A  whirlwind  is  noiseless.  Let,  however, 
the  great  billows  strike  a  tree  and  wrench  it  from 
the  ground,  and  we  can  hear  the  secondary  shorter 
waves  which  set  out  from  the  struggling  limbs  and 
the  tossing  leaves. 

Our  unconsciousness  is  no  proof  of  the  absence 
of  sound.  There  are,  doubtless,  sounds  in  Nature 
of  which  we  have  no  conception.  Could  our  sense 
be  quickened,  what  celestial  harmony  might  thrill 
us  !  Professor  Cooke  beautifully  says  :  "  The  very 
air  around  us  may  be  resounding  with  the  halle- 
lujahs of  the  heavenly  host,  while  our  dull  ears 
hear  nothing  but  the  feeble  accents  of  our  broken 
prayers." 

The  ability  of  the  ear  to  detect  and  analyze  sound 
is  wonderful  beyond  all  comprehension.  Sound- 
waves chase  each  other  up  and  down  through  the 
air,  superposed  in  entangled  pulsations,  yet  a  cylin- 
der of  the  air  not  larger  than  a  quill  conveys  them 
to  the  ear,  and  each  string  of  that  wonderful  harp 
selects  its  appropriate  sound,  and  repeats  the  music 
to  the  soul  within.  Though  a  thousand  instruments 
be  played  at  once,  there  is  no  confusion,  but  each  is 
heard,  and  all  blend  in  harmony. 

THE  TENDENCY  OF  NATURE  TO  Music. — "  Friction," 
says  Tyndall,  "  is  rhythmic."  A  bullet  flying  through 


j  g  2  NA  TURAL  PHIL  OSOPHY. 

the  air  sings  softly  as  a  bird.  The  limbs  and  leaves 
of  trees  murmur  as  they  sway  in  the  breeze.  The 
rumble  of  a  great  city,  all  the  confused  noises  of  na- 
ture when  softened  by  distance,  are  said  to  be  on  one 
pitch — the  key  of  F.  Falling  water,  singing  birds, 
sighing  winds,  everywhere  attest  that  the  same  Di- 
vine love  of  the  beautiful  which  causes  the  rivers 
to  wind  through  the  landscape,  the  trees  to  bend  in 
a  graceful  curve — the  line  of  beauty — and  the  rarest 
flowers  to  bud  and  blossom  where  no  eye  save  His 
may  see  them,  delights  also  in  the  anthem  of  praise 
which  Nature  sings  for  His  ear  alone. 

SENSITIVE  FLAMES. — Flames  are  frequently  ex- 
tremely sensitive  to  certain  sounds.  At  an  instru- 
mental concert  the  gaslights  often  vibrate  in  unisoa 
with  certain  pulsations  of  the  sound  which  they  seem 
to  select.  This  is  most  noticeable  when  the  pres- 
sure of  gas  is  so  great  that  the  flame  is  just  on  the 
verge  of  flaring,  and  the  vibration  of  the  sound- 
wave is  sufficient,  as  it  were,  to  "  push  it  over  the 
precipice."  If  we  turn  on  the  gas,  in  a  common 
fish-tail  burner,  we  reach  a  point  where  a  shrill 
whistle  will  produce  the  same  effect  as  increased 
pressure  of  the  gas,  and  cause  the  jet  to  thrust  out 
long,  quivering  flames.  Prof.  Barret,  of  London,  de- 
scribes a  peculiar  jet  which  was  so  "  sensitive  thai 
it  would  tremble  and  cower  at  a  hiss,  like  a  human 
being,  and  even  beat  time  to  the  ticking  of  a 
watch." 

SINGING  FLAMES. — If  we  lower  a  glass  tube  over  a 


ACOUSTICS. 


Fig.  130. 


small  jet  of  gas,  we  soon  reach  a  point  where  the 
flame  leaps  spontaneously  into  song.  At  first  the 
sound  seems  far  re- 
mote, but  gradually 
approaches  until  it 
bursts  into  a  shrill 
scream  that  is  al- 
most intolerable. 
The  length  of  the 
tube  and  the  size 
of  the  jet  determine 
the  pitch  of  the 
note.  If  we  raise 
the  tube  to  a  point 
where  it  is  just  ready 
to  sing,  we  shall  find 
that  it  will  respond 
to  the  voice  when 
the  proper  note  is 
struck.* 

The  flame,  owing 
to  the  friction  at  the 
mouth  of  the  pipe, 
is  thrown  into  vibration.  The  air  in  the  tube,  being 
heated,  rises,  and  not  only  vibrates  in  unison  with 
the  jet,  but,  like  the  organ-pipe,  selects  the  tone 
which  is  adapted  to  its  length,  and  in  part  governs 
the  pulsations  of  the  flame. 

*  See  Rev.  Chem.,  p.  55.  The  jets  are  made  by  drawing  out 
glass  tubing  to  a  fine  point  over  a  spirit-lamp.  The  length  of  the 
tube  may  be  varied,  as  in  the  figure,  by  means  of  a  paper,  tube. 


Tg4  NATURAL  PHILOSOPHY. 

Practical  Questions.— -1.  Why  cannot  the  rear  of  a  long  column  of  sol. 
diers  keep  time  to  the  music  in  front  ?  2.  Three  minutes  elapse  between  the 
flash  and  report  of  a  thunderbolt;  how  far  distant  is  it?  3.  Five  seconds 
expire  between  the  flash  and  report  of  a  gun  ;  what  is  its  distance  ?  4.  Sup- 
pose a  speaking-tube  should  connect  two  villages  ten  miles  apart ;  how  long 
would  it  take  the  sound  to  travel  that  distance  ?  5.  The  report  of  a  pistol- 
ehot  was  returned  to  the  ear  from  the  face  of  a  cliff  in  four  seconds  ;  what 
was  the  distance  ?  6.  What  is  the  cause  of  the  difference  between  the  voice 
of  man  and  woman  ?  A  base  and  a  tenor  voice  ?  7.  What  is  the  number  of 
vibrations  per  second  necessary  to  produce  the  fifth  tone  of  the  scale  of  C  ? 
8.  What  is  the  length  of  each  sound-wave  in  that  tone  when  the  temperature 
is  at  zero  ?  9.  What  is  the  number  of  vibrations  in  the  fourth  tone  above 
middle  C  ?  1 0.  A  meteor  of  Nov.  13, 1868,  is  said  to  have  exploded  at  a  height 
of  60  miles  ;  what  time  would  it  have  required  for  its  sound  to  reach  the  earth  ? 
1 1 .  A  stone  was  let  fall  into  a  well,  and  in  four  seconds  was  heard  to  strike 
the  bottom ;  how  deep  was  the  well  ?  12.  What  time  will  it  require  for  a 
sound  to  travel  five  miles  in  the  still  water  of  a  lake  ?  13.  How  much  louder 
will  be  the  report  of  a  gun  to  an  observer  at  a  distance  of  20  rods  than  to  one 
at  half  a  mile  ?  14.  Does  sound  travel  faster  at  the  foot  or  at  the  top  of  a 
mountain  ?  15.  Why  is  an  echo  weaker  than  the  original  sound  ?  16.  Why 
is  it  eo  fatiguing  to  talk  through  a  speaking-trumpet  ? 


Curious  Facts  in  Sound.—  Silliman  says  the  roar  of  cannon  has  been  heard 
at  a  distance  of  250  miles  by  putting  the  ear  to  the  ground.  In  Capt.  Parry's 
third  Expedition,  Lieut.  Forster  carried  on  a  conversation  with  a  man  at  a 
distance  of  1}  miles.  The  sentinel's  "All's  well"  has  been  heard  from  Old  to 
New  Gibraltar,  a  distance  of  10  miles.  The  cannonading  at  the  battle  of  Jena 
was  heard  at  Dresden,  92  miles  away.  The  celebrated  echo  of  the  Metelli  a< 
Rome  was  capable  of  repeating  the  first  line  of  the  JSneid  8  times  distinctly 
In  Fairfax  County,  Va.,  is  an  echo  which  will  return  20  notes  played  on  a 
flute,  but  supplies  the  place  of  some  notes  with  their  thirds,  fifths,  or  octaves. 
Sir  John  Herschel  says  the  tick  of  a  watch  may  be  heard  from  one  end  of  the 
Abbey  Church  of  St.  Albans  to  the  other.  At  Carisbrook  Castle,  in  the  Isle 
of  Wight,  is  a  well  210  feet  deep  and  12  feet  wide.  It  is  lined  with  smooth 
masonry.  When  a  pin  is  dropped  into  the  well  it  is  distinctly  heard  to  strike 
the  water.  In  certain  parts  of  the  Colosseum  at  London  the  tearing  of  paper 
sounds  like  the  patter  of  hail,  while  a  single  exclamation  comes  back  a  peal 
of  laughter. 

A  tired  bee  hums  on  E,  while  in  pursuit  of  honey  it  hums  contentedly  on 
A.  The  common  horse-fly,  when  held  captive,  moves  its  wings  335  times  a 
second ;  a  honey-bee,  190  times. 

Youmans  says  it  is  marvellous  how  slight  an  impulse  throws  a  vast  amount 
of  air  into  motion.  We  can  easily  hear  the  song  of  a  bird  500  feet  above  us. 
For  its  melody  to  reach  us  it  must  have  filled  with  wave-pulsations  a  sphere 
of  air  1,000  feet  in  diameter,  or  set  in  motion  18  tons  of  the  atmosphere. 


n    fight. 


The  sunbeam  comes  to  the  earth  as  simply  motion  of  ether- waves,  yet  it  is 
the  only  source  of  beauty,  life,  and  power.  In  the  growing  plant,  the  burning 
coal,  the  flying  bird,  the  glaring  lightning,  the  blooming  flower,  the  rushing 
engine,  the  roaring  cataract,  the  pattering  rain— we  see  only  vailed  manifesta- 
tions of  this  one  all-energizing  fore*. 


OPTICS. 

OPTICS  treats  of  Light. 

DEFINITIONS. — A  luminous  body  is  one  that  emits 
light.  A  non-luminous  body  is  one  that  reflects 
light,  and  is  visible  only  in  the  presence  of  a  lumi- 
nous body.  A  medium  is  any  substance  through 
which  light  passes.  A  transparent*  body  is  one  that 
offers  so  little  obstruction  to  the  passage  of  light 
that  we  can  see  objects  through  it.  A  translucent 
body  is  one  that  lets  some  light  pass,  but  not 
enough  to  render  objects  visible  through  it.  An 
opaque  body  is  one  that  does  not  transmit  light.  A 
ray  of  light  is  a  single  line  of  light ;  it  may  be  traced 
in  a  dark  room  into  which  a  sunbeam  is  admitted. 

*  Though  we  speak  of  transparent  and  opaque  substances, 
these  terms  are  merely  relative.  No  body  is  perfectly  transpar- 
ent,  nor  is  any  entirely  opaque.  Glass  obstructs  some  light. 
It  is  said  that  if  the  atmosphere  were  700  miles  deep,  no  light 
would  reach  our  eye.  Deep-sea  dredging  has  recently  shown 
that  light  penetrates  water  to  great  depths,  BO  that  even  the 
Atlantic  cable  may  not  lie  in  an  abyss  of  utter  darkness.  On  the 
other  hand,  gold,  when  beaten  into  leaf,  becomes  translucent, 
and  appears  of  a  faint  green  color  j  and  horn,  when  scraped 
becomes  semi-transparent. 


1 8 8  NA  TURAL  PHIL OSOPHT. 

by  the  floating  particles  of  dust  which  reflect  the 
light  to  the  eye. 

THE  VISUAL  ANGLE  is  the  angle  formed  at  the  eye 
by  rays  coming  from  the  extremities  of  an  object. 
The  angle  A  O  B  is  the  angle  of  vision  subtended 

Fig.  131. 


by  the  object  A  B.  The  size  of  this  angle  varies 
with  the  distance  of  the  body.  A  B  and  A'  B'  are 
of  the  same  length,  and  yet  the  angle  A'  O  B'  is 
much  smaller  than  A  O  B,  and  hence  A'  B'  will 
seem  much  shorter  than  A  B.  We  estimate  the  dis- 
tance and  size  of  objects  in  various  ways,  but  the 
two  are  intimately  connected,  since  we  have  by 
long  experience  learned  to  associate  them.  Know- 
ing the  distance  of  an  object,  we  determine  its  size 
immediately  from  the  visual  -angle.  We  can  vary 
the  apparent  size  of  any  body  at  which  we  are  look- 
ing, if  by  any  means  we  increase  or  diminish  this 
angle.  This  principle  will  be  found  of  great  impor- 
tance in  the  formation  of  images  by  mirrors  and 
lenses. 

LAWS  OF  LIGHT. — 1.  Light  passes  off  from  a  lumi- 
nous body  equally  in  every  direction. 

2.  Light  travels  through  a  uniform  medium  in 
straight  lines. 


OPTICS.  !89 

3.  The  intensity  of  light  decreases  as  the  square 
of  the  distance  increases. 

THE  TELOCITY  OF  LIGHT  is  about  183,000  miles 
per  second.  This  is  so  great  that  for  all  distances 
on  the  earth  it  is  instantaneous.  A  sunbeam  would 
girt  the  globe  quicker  than  we  can  wink.  This  rate 
has  been  determined  in  various  ways,  but  a  most 
simple  method  is  thus  explained.  The  planet  Ju- 
piter has  four  moons;  as  these  pass  around  the 
planet,  they  are  eclipsed  from  our  sight  at  regular 
intervals.  In  the  cut,  let  J  represent  Jupiter,  e  one 

Fig.  138. 


of  the  moons,  S  the  sun,  and  T  and  t  different  posi- 
tions of  the  earth  as  it  moves  in  its  orbit  around  the 
sun.  Homer  noticed  that  when  the  earth  was  at  T, 
the  eclipse  occurred  16  inin.  and  36  sec.  earlier  than 
when  at  t.  He  could  account  for  this  only  on  the 
supposition  that  it  requires  that  time  for  the  light 
to  travel  across  the  earth's  orbit.  This  distance  is 
183,000,000  miles.  Hence  the  velocity  is  about 
183,000  miles  per  second. 

THE  UNDULATORY  THEORY  OF  LIGHT. — There   is 
supposed  to  be  a  fluid,  termed  ether,  constituting  a 


j  p0  NA  TURAL  PHIL  OSOPHY. 

kind  of  universal  atmosphere,  diffused  throughout  all 
space.  It  is  so  subtle  that  it  fills  the  pores  of  all 
bodies,  eludes  all  chemical  tests,  passes  in  through 
•  the  glass  receiver  and  remains  even  in  the  vacuum 
of  an  air-pump.  A  luminous  body  sets  in  motion 
waves  of  ether,  which  pass  off  in  every  direction. 
These  move  at  the  rate  of  183,000  miles  per  second, 
and  breaking  upon  the  eye,  give  to  us  the  impres- 
sion of  sight.  This  etherial  wave-motion  is  pre- 
cisely like  that  of  sound,  except  that  the  vibrations 
are  transverse  (crosswise)  to  the  line  of  direction. 
Thus,  if  we  suppose  a  star  directly  overhead  and  a 
ray  of  light  coming  down  to  us,  we  should  conceive 
that  the  particles  which  compose  the  waves  are 
vibrating  N.  S.  E.  W.,  and  toward  every  other  point 
of  the  compass  all  at  once. 

KEFLECTION  OF  LIGHT. 

DIFFUSED  AND  EEFLECTED  LIGHT. — When  light 
falls  upon  any  surface,  one  portion  is  transmitted 
and  another  is  reflected.  The  law  is  that  of  Motion 
and  Sound — i.  e.,  "  The  angle  of  incidence  is  equal  to 
the  angle  of  reflection."  When  the  surface  is 
rough,  the  multitude  of  little  protuberances  scatter 
the  rays  in  every  conceivable  direction,  and  we  can 
therefore  see  such  a  body  from  any  point.  This 
forms  what  we  term  diffused  light.  When  the  surface 
is  smoothed  and  polished,  the  rays  are  more  uni- 
formly reflected  in  particular  directions,  and,  when 
we  stand  in  the  proper  position,  will  bring  to  us  the 


OPTICS. 


images  of  other  objects.  Wo  thus  view  all  non- 
luminous  objects  by  means  of  irregularly  reflected 
(diffused)  light,  and  images  of  objects  by  means 
of  regularly  reflected  light.  However,  the  most 
perfectly  polished  substance  diffuses  some  light  —  • 
enough  to  enable  us  to  trace  its  surface  ;  were  it  not 
so,  we  could  not  be  aware  of  its  existence.  The  dej 
ception  of  a  mirror  is  oftentimes  very  nearly  com- 
plete ;  yet  a  little  dust  or  vapor,  increasing  the  irregu- 
lar reflection,  will  at  once  bring  the  surface  to  view. 

EEFLECTION  VARIES  WITH  THE  ANGLE.  —  We  notice 
this  very  clearly  if  we  look  at  the  images  of  objects 
in  still  water.  Those  which  are  near  us  are  not  as 
distinct  as  those  on  the  opposite  bank,  because  the 
rays  from  the  latter  strike  the  water  more  obliquely 
than  the  former,  and  so  are  more  perfectly  reflected 
to  the  eye.  The  image  of  the  sun  at  mid-day  is  not 
so  bright  as  when  it  is  near  the  horizon. 

MIRRORS.  —  All  reflecting  surfaces  are  termed  mir- 
rors. These  are  of  three  kinds  —  plane,  concave,  and 
convex.  The  first  has  a  flat  surface  ;  the  second, 
one  like  the  inside,  and  the  third,  one  like  the  outside 
of  a  watch-crystal. 

The  general  principle  of  mirrors  is,  that  the  image 
is  always  seen  in  the  direction  of  the  reflected  ray  as  it. 
enters  the  eye. 

PLANE  MIRRORS.  —  Bays  of  light  retain  their  rela- 
tive direction  after  reflection  from  a  plane  surface.* 

*  The  effect  of  the  various  mirrors  is  .best  understood  if  we 
draw  a  mirror  of  the  kind  under  consideration,  and  then  repre- 


!  ^  2  -^  TTJRAL  PHIL OSOPHY. 

An  image  seen  in  a  plane  mirror  is  therefore  erect 
and  of  the  same  size  as  the  object.  It  is,  however, 
reversed  right  and  left. 

Why  the  image  is  seen  as  far  behind  the  mirror  as 
the  object  is  in  front  of  it. — Let  A  B  be  an  arrow  held 
Fig.  133.  in  front  of  the  mirror, 

MN.  Bays  of  light 
from  the  point  A 
striking  upon  the 
mirror  at  C,  are  re- 
flected, and  enter  the 
eye  as  if  they  came 
from  a.  Bays  from  B, 
in  the  same  manner,  seem  to  come  from  b.  Since 
the  image  is  seen  in  the  direction  of  the  reflected 
rays,  it  appears  at  a  b,  a  point  as  far  behind  M  N 
as  the  real  arrow  is  in  front  of  it. 

Why  we  can  see  several  images  of  an  object  in  a  mir- 
ror.— Metallic  mirrors  form  but  a  single  image.  If, 
however,  we  look  obliquely  at  the  image  of  a  candle 


sent  rays  of  the  different  classes,  erect  perpendiculars  at  the 
point  of  incidence  and  find  the  reflected  rays.  A  little  prac- 
tice of  this  kind  will  benefit  more  than  any  description.  It  will 
aid  in  drawing  the  perpendicular  to  a  convex  or  concave  sur- 

.  face,  if  we  remember  that  it  is  always  a  radius  of  the  sphere 
of  which  the  mirror  forms  a  part.  A  book  held  in  various  posi- 
tions before  a  common  looking-glass  should  be  used  to  illustrate 
the  action  of  plane  mirrors.  Many  of  the  effects  of  concave  and 
convex  mirrors  may  be  seen  on  the  inner  and  outer  surface  of  a 

v  bright  spoon,  a  call-bell,  or  a  metal  cup.  Much  instructive 
amusement  may  be  obtained  in  the  examination  of  the  curious 
and  grotesque  figures  thus  revealed. 


OPTICS. 


193 


in  a  looking-glass,  we  shall  often  see  several  images, 
the  first  one  very  feeble,  the  next  bright,  and  the 
others  gradually  diminishing  in 
intensity.  The  ray  from  A  is  in 
part  reflected  to  the  eye  from  the 
glass  at  5,  and  gives  rise  to  the 
image  a ;  the  remainder  passes 
on  and  is  reflected  from  the  me- 
tallic surface  at  c,  and  coming  to 
the  eye  forms  a  second  image  a. 
The  ray  c  c£,  when  leaving  the  glass  at  d,  loses  a  part, 
which  is  reflected  back  to  form  a  third  image.  This 
ray  in  turn  is  divided  to  form  a  fourth,  and  so  on. 

Fig.  135. 


Images  seen  in  water  are  symmetrical,  but  inverted. 
The  reason  of  this  is  best  understood  by  holding  an 


!  94  NA  TTJRAL  PHILOSOPHY. 

object  in  front  of  a  horizontal  looking-glass  and  no- 
ticing the  angle  at  which  the  various  rays  must  strike 
the  surface  in  order  to  be  reflected  to  the  eye.  When 
the  sun  or  moon  is  shining  high  in  the  heavens,  we 
always  see  the  image  in  the  water  at  only  one  spot, 
while  the  rest  of  the  surface  appears  dark.  The  light 
falls  upon  all  parts,  but  the  rays  are  reflected  at  the 
right  angle  to  reach  the  eye  from  one  point  alone. 
Each  observer  sees  the  image  at  a  different  place. 
When  the  surface  of  the  water  is  ruffled,  a  long  tremu- 
lous line  of  light  is  reflected  from  the  side  of  each  tiny 
wave  that  is  turned  toward  us.  As  each  little  billow 
rises,  it  flashes  a  gleam  of  light  to  our  eye,  and  then 
sinking,  comes  up  beyond,  only  to  reflect  another  ray. 
A  CONCAVE  MIRROR  tends  to  collect  the  rays  of  light. 
— The  point  where  the  reflected  rays  meet  is  termed 
the  principal  focus  (focus,  a  hearth).  It  is  half  way  be- 
tween the  mirror  and  the  centre  of  curvature — i.  e., 
the  centre  of  the  hollow  sphere  of  which  the  mirror 
is  a  part.  In  Fig.  136  we  have  parallel  rays  falling 
upon  a  mirror.  C  is  the  centre  of  curvature ;  F, 
the  principal  focus,  half-way  between  A  and  C  ;  A  F, 
Fig.  136.  the  focal 

distance; 
C  B,  C  D, 
etc.,  radii  of 
the  sphere 
(perpendic- 
ulars, to  find  the  angle  of  incidence) ;  the  angles 
H  B  C,  G  D  C,  etc.,  equal  respectively  to  F  B  C, 


OPTICS. 


'95 


F  D  C,  etc.  A  light  held  at  C  will  have  its  rays 
brought  to  a  focus  at  C  ;  on  the  other  hand,  one  at 
F  will  be  reflected  in  a  beam  of  parallel  rays. 

Images  formed  by  concave  mirrors. — Hang  a  con- 
cave mirror  against  the  wall,  and  stand  closely  to  it 
between  the  mirror  and  the  principal  focus.  The 
image  is  erect  and  much  larger  than  life.  The  ray  a 
falls  upon  the  mirror,  is  reflected  and  strikes  the 

Fig.  137. 


eye  as  if  it  came  from  A.  In  the  same  manner  b  is 
seen  at  B.  The  visual  angle  is  increased  the  nearer 
we  approach  the  mirror,  and  hence  the  greater  the 
magnifying  power.  We  now  walk  back.  When  we 
reach  the  focus,  the  image  becomes  blurred,  and 
finally  disappears.  We  are  in  the  position  of  the 
candle  a  b  (Fig.  138),  and  the  real  image  is  behind 


196 


NATURAL  PHILOSOPHY. 


us  at  A  B.  A  few  of  the  parallel  rays,  however, 
enter  the  eye,  and  an  indistinct  image  is  formed. 
Retiring  still  farther,  we  come  to  the  centre  of  curva- 
ture. Here  we  find  no  distinct  image,  although 
portions  of  our  figure,  as  we  catch  snatches  of  the 
rays  which  are  forming  the  image  A  B,  are  magnified 
in  the  most  uncouth  and  absurd  manner.  As  we 

Fig.  138. 


continue  to  recede,  we  reach  a  point  beyond  the 
centre  of  curvature.  Here  we  occupy  the  position 
A  B  (Fig.  138),  and  we  see  the  image  in  the  position 
a  b,  in  front  of  the  mirror  inverted.  It  is  inverted 
since  the  rays  cross  at  the  focus,  and  is  smaller  than 
life  because  the  visual  angle  is  diminished.  The 
positions  occupied  by  the  two  candles,  a  b  and  A  B, 
are  termed  conjugate  foci,  since  an  object  at  either 
point  is  brought  to  a  focus  at  the  other. 

fe.  139.  A  CONVEX  MIR- 

ROR tends  to  scatter 
the  rays  of  light. — 
In  the  figure  we 
notice  how  the 
parallel  rays  A  D 
and  B  K  are  reflected  in  the  diverging  lines  D  E 


OPTICS.  KJJ 

and  K  H.  An  eye  receiving  these  rays  will  perceive 
the  image  of  A  B  at  a  b,  erect,  and  smaller  than  the 
object  since  the  visual  angle  is  diminished. 

TOTAL  EEFLECTION. — When  we  look  very  obliquely 
into  a  pond,  we  .cannot  see  the  bottom,  becausp 

the   rays   of   light   from   below  Fig.  140. 

are  reflected  downward  at  the 
surface.  If  we  look  up  into  a 
glass  of  water,  we  shall  see  the 
upper  surface  gleaming  like  bur- 
nished silver.  This  effect  occurs 
only  when  light  passes  at  a 
definite  angle  from  a  denser  to  a 
rarer  medium.  It  is  termed  total,  since,  unlike  the 
other  cases  of  reflection,  all  the  light  is  bent  back. 

KEFRACTION  OF  LIGHT. 

We  have  already  seen  that  when  a  ray  of  light 
passes  from  one  medium  to  another  of  different 
density,  one  portion  is  irregularly  reflected,  and  by 
that  means  the  surface  is  made  visible ;  that,  if  the 
surface  is  smooth,  the  larger  part  is  regularly  re- 
flected, and  in  that  way  images  of  objects  are  seen. 
We  now  speak  of  the  portion  which  pscsses  on  to  the 
next  medium,  and  which  is  refracted  or  bent  out  of 
its  course. 

ILLUSTRATIONS  OF  REFRACTION. — A  spoon  in  clear 
tea  appears  bent.  An  oar  dipping  in  still  water 
seems  to  break  at  the  point  where  it  enters  the 
water.  Fish  seem  nearer  the  surface  than  they 


198 


NATURAL  PHILOSOPHY. 


really  are,  and  Indians,  who  spear  them,  always  try 
to  strike  perpendicularly,  or  else  aim  lower  than 
they  apparently  lie.  Water  is  always  deeper  than 
it  appears.  Look  obliquely  and  steadily  into  a  pail 
of  water,  then  place  your  finger  on  the  outside 
where  the  bottom  seems  to  be ;  you  will  be  sur- 
prised, on  examination,  to  find  that  the  real  bottom 
is  several  inches  below  your  finger.  Fill  a  glass 
dish  with  water,  and,  darkening  the  windows,  let  a 
single  sunbeam  fall  upon  the  surface.  The  ray  will 
be  seen  to  bend  as  it  enters.  A  little  chalk-dust 
scattered  through  the  air  will  make  the  beam  very 
Fig.  i4i.  distinct.  Place  a 

<*8^**v  cent  at  the  bottom 
of  a  bowl.  Standing 
where  you  cannot 
see  the  coin,  let  an- 
other person  pour 
water  into  the  ves- 
sel, when  the  coin 
will  be  apparently 
lifted  into  view.  Let  L,  Fig.  142,  be  a  body  be- 
neath the  water.  A  ray,  L  A,  coming  to  the  surface, 
is  bent  downward  toward  C,  and  strikes 
the  eye  as  if  it  came  from  L'.  The  ob- 
ject will  therefore  be  elevated  above 
its  true  place.  In  order  to  understand 
the  apparent  change  of  position  pro- 
duced by  refraction,  we  have  but  to  re- 
member this  principle — the  object  is 


Fig.  142. 


OPTICS. 


199 


Fig.  143. 


always  seen  in  the  direction  of  the  refracted  ray  as  it 
enters  the  eye.' 

LAWS  OF  BEFRACTION. — 1.  In  passing  into  a  rarer 
medium,  the  ray  is  bent  from  the  perpendicular.  2. 
In  passing  into  a  denser  medium,  the  ray  is  bent 
toward  the  perpendicular. 

PATH  OF  BAYS  THROUGH  A  WINDOW-GLASS. — When  a 
ray  enters  a  window-glass,  it  is  refracted  toward  the 
perpendicular  (2d  law),  and  on 
leaving,  is  equally  refracted 
from  the  perpendicular  (1st 
law).  The  general  direction 
of  objects  is  therefore  not 
changed.  A  poor  quality  of 
glass,  however,  produces  dis- 
tortion by  its  unequal  density 
and  uneven  surface. 

PATH   OF   RAYS    THROUGH   A 

PRISM. — A  ray  of  light,  on  entering  and  leaving  a 
prism,  is  re- 
fracted as  in 
the  case  of  a 
window-glass. 
The  inclina- 
tion of  the 
sides,  howev- 
er, causes  the 
ray  to  be  bent 
twice  in  tJie  same  direction. 
fore  appear  to  be  at  r. 


Fig.  144. 


The  candle  L  will  there- 


2OG 


NATURAL  PHILOSOPHY. 


LENSES. — A  lens  is  a  transparent  body,  with  at 
least  one  curved  surface.  There  are  two  general 
classes  of  lenses,  concave  and  convex.  Six  varieties 
of  these  are  used  in  optical  instruments,  viz.,  the 


Fig.  145. 


double-convex  (M),  the  plano-convex  (N),  the  me- 
niscus (O),  the  double-concave  (P),  the  plano-con- 
cave (Q),  and  the  concavo-convex  (R). 

THE  DOUBLE-CONVEX  LENS  has  two  convex  surfaces. 
Rays  of  light  falling  upon  a  convex  lens,  as  upon  a 
concave  mirror,  tend  to  converge  to  a  focus.  A  ray 

Fig.  146. 


passing  along  the  line  X  (the  axis  of  the  lens),  as  it 
strikes  the  surface  perpendicularly,  is  not  refracted. 
The  parallel  rays  M,  L,  etc.,  are  refracted  both  on 
entering  and  on  leaving  the  lens,  and  are  brought 
together  at  F,  the  focus.  If  a  light  were  placed 
at  F,  its  rays  would  be  refracted  in  parallel  lines. 


OPTICS.  201 

The  convex  lens  is  sometimes  termed  a  burning- 
glass.  It  is  used,  like  the  concave  mirror,  for 
collecting  the  sun's  rays,  and  hence  is  a  ready 
means  of  obtaining  fire.  Lenses  have  been  manu- 
factured of  sufficient  power  to  fuse  the  metals. 
One,  of  two  feet  in  diameter,  made  at  Leipsic  in  1691, 
melted  plate-iron,  and  converted  a  piece  of  burnt 
brick  into  yellow  glass.  The  image  formed  by  a  con- 
vex lens  is,  in  size  and  position,  precisely  like  that 

Fig.  147. 


seen  in  a  concave  mirror.  If  we  hold  a  lens  above 
a  printed  page,  when  we  obtain  the  focal  distance 
correctly,  we  shall  find  the  letters  right-side  up  and 
highly  magnified.  By  an  inspection  of  the  figure  we 
see  that  the  converging  power  of  the  lens  simply  in- 
creases the  visual  angle,  and  thus  makes  the  object 
A  B  appear  the  size  a  b.  Moving  the  lens  back  from 
the  page,  the  letters  disappear  entirely  as  we  pass 
the  principal  focus.  At  length  they  suddenly  reap- 
pear again,  but  smaller  and  inverted.  By  examining 
Fig.  148,  we  see  how  the  rays  from  A  B  cross  each 
other  at  the  focus,  and  thus  invert  the  image  a  6,  at 
the  same  time  reducing  the  visual  angle. 

9* 


202 


NATURAL  PHILOSOPHY. 
Fig.  148. 


THE  DOUBLE-CONCAVE  LENS  has  two  concave  sur- 
faces. Rays  of  light  falling  upon  a  double  concave 
lens,  as  upon  a  convex  mirror,  are  scattered.  Thus, 

Fig.  149. 


diverging  rays  from  a  light  at  L  are  rendered  more 
diverging ;  and,  to  an  eye  which  receives  the  rays 
M  N,  the  candle  would  be  located  at  I,  where  the 
image  would  be  seen. 

The  image  formed  by  a  concave  lens  is,  in  size  and 
position,  like  that  seen  in  a  convex  mirror.  The 
visual  angle  is  decreased,  and  the  rays  do  not  cross ; 
hence  the  image  of  A  B  is  seen  at  a  6,  erect,  and 
diminished  in  size  (Fig.  150). 

MIKAGE  is  an  optical  delusion  whereby  pictures  of 
distant  objects  are  seen  as  if  near.  On  the  heated 
deserts  of  Africa,  the  traveller  beholds  quiet  lakes 
and  shadows  of  trees  in  their  cool  waters.  Bushing 


203 


forward  to  slake  his  eager  thirst,  he  finds  only  the 
barren  waste  of  sand.  The  mariner  often  sees  in 
the  sky  the  images  of  ships,  and  the  far-distant 
coast,  with  its  familiar  cliffs  and  shipping,  so  perfect 
in  outline  as  to  be  instantly  recognized. 

The  cause  is  found  in  the  reflection  and  refraction 
of  the  rays  of  light  as  they  traverse  layers  of  air  of 
unequal  density.  In  Fig.  151,  rays  of  light  from  a 
clump  of  trees  at  the  left  are  reflected  from  a  layer 
at  a,  and  enter  the  eye  of  the  Arab  as  if  they  came 
from  the  surface  of  a  lake  below.  The  sandy  desert 
itself,  shimmering  in  the  hot  sun,  looks  in  the  dis- 
tance not  unlike  the  surface  of  tranquil  water. 
Sometimes,  at  sea,  a  layer  of  air  high  up  in  the  sky 
acts  as  a  total  reflector,  and  sends  down  an  inverted 
image  of  ships  which  are  far  beyond  the  horizon. 


204 


NATURAL  PHILOSOPHY. 
Fig.  151. 


THE  COMPOSITION  or  LIGHT. 

SOLAR  SPECTRUM. — When  a  sunbeam  is  allowed 
to  shine  through  a  prism,  the  ray  is  not  only  bent 
from  its  course,  as  we  have  already  seen,  but  is 
spread  out,  fan-like,  into  a  broad  band  of  rain- 
bow-colors, called  the  solar  spectrum.  It  contains 
the  seven  primary  colors — Violet,  Indigo,  Blue, 
Green,  Yellow,  Orange,  Red.  (These  may  be  re- 
membered in  their  order,  by  noticing  that  the  initial 
letters  spell  the  absurdly  meaningless  word,  Vib-gy- 
or.)  If  we  receive  the  spectrum  on  a  concave  mir- 
ror, or  pass  it  through  a  double-convex  lens,  it  will 
form  a  white  spot.  We  therefore  conclude  that 
white  light  is  composed  of  seven  different  colors.* 

*  Many  hold,  with  Sir  David  Brewster,  that  there  are  but  three 
primary  colors — red,  yettow,  and  blue.  It  is  often  convenient  for 
purposes  of  explanation  to  thus  consider  it.  Helmholtz  denies*  the 


OPTICS.  205 


They  are  separated  because  the  prism  bends  them 
unequally.     The  violet,  is  most  refracted  and  the  red 


Fig.  152 


least.  The  dispersive  power  of  a  prism,  i.  e.,  its  abil- 
ity to  spread  apart  the  colored  rays,  depends  on  the 
material  of  which  it  is  made.  A  flint-glass  prism  is 

truth  of  this  in  toto,  although  he  admits  that  all  colors  can  be 
produced  from  the  three.  On  the  other  hand,  John  Herschel 
claims  to  have  discovered  an  eighth  color  below  the  red,  which 
is  of  a  crimson  hue,  and  a  ninth  beyond  the  violet,  which  is  of  a 
lavender  hue.  Professor  Stokes  in  addition  believes  in  a  tenth 
color  beyond  the  lavender,  which  he  styles  the  fluorescent  ray,  as 
it  resembles  the  shade  of  some  kinds  of  fluor  spar. 


206  NATURAL  PHILOSOPHY. 

commonly  used.  A  hollow  one,  filled  with  oil  oi 
cassia  or  bisulphide  of  carbon,  has  far  higher  dis- 
persive power. 

THKEE  CLASSES  OF  BAYS  IN  THE  SOLAR  SPECTRUM. — 
These  are  the  calorific,  or  heat-rays ;  the  colorific,  or 
luminous  rays;  and  the  actinic,  or  chemical  rays. 
If  we  examine  the  spectrum  with  a  delicate  ther- 
mometer, we  shall  find  that  the  heat  increases  grad- 
ually from  the  violet  toward  the  red,  and  becomes 
the  greatest  in  the  dark  space  just  beyond.  If  we 
test  with  a  paper  containing  nitrate  of  silver,  it  will 
blacken  least  in  the  red,  more  toward  the  violet,  and 
most  in  the  dark  space  beyond.  Artificial  lights 
differ  in  the  proportion  of  the  three  classes  of  rays. 
Seeds  will  sprout  best  under  a  blue  glass.  Red  is 
the  warmest  color.  A  photograph  could  be  taken  in 
the  dark  by  means  of  the  chemical  rays  alone.  A 
soldier  dressed  in  gray  or  green  clothing  is  less  lia- 
ble to  be  shot  than  one  in  red  or  yellow. 

COMPLEMENTARY  COLORS. — Two  colors,  which  by 
their  mixture  produce  white  light,  are  termed 
complementary  to  each  other.  Let  us  suppose, 
for  simplicity  of  statement,  that  white  light  is 
composed  of  the  three  colors,  red,  yellow,  and  blue. 
Then,  since  yellow  and  blue,  when  mixed,  form 
green,  we  have  red  and  green  as  complementary 
colors.  Eed  and  blue  produce  violet ;  hence  yel- 
low and  violet  are  complementary  colors.  If  we 
look  steadily  at  a  colored  wafer  lying  on  a  sheet  of 
white  paper,  we  shall  see  a  fringe  of  the  comple- 


OPTICS. 


207 


GREEN 


liOW 


VIOLE1 


ORANGE 


EORHORttK 


RED 


mentary  color  play-  Fig.  153. 

,       V  .v        -r*     '  GREEN 

ing  about  it.  If  we 
watch  bright  red 
clouds,  the  patches 
of  clear  sky  will  seem 
green.  In  examining 
'ribbons  of  the  same 
color,  the  eye  be- 
comes wrearied  and 
unable  to  detect  the 
shade,  because  of  the 
mingling  of  the  complementary  color.  A  knowledge 
of  this  subject  is  very  essential  to  the  harmony  of 
colors  in  painting,  or  the  arrangement  of  a  bouquet, 
that  the  result  may  be  pleasing  to  a  cultivated  taste. 
INTERFERENCE  OF  LIGHT. — Newton's  Rings. — The 
convex  side  of  a  plano-convex  lens  is  pressed  down 
upon  a  flat  surface  of  glass.  Mg  154 

-Lne  two  surfaces  will  toucn  "^~ — __^__^^  w///^&yt>i*^ 
each  other  at  the  centre ;  and  "^ 
if  different  circles  be  described  around  this  point, 
at  all  parts  of  each  circle  the  two  surfaces  will  be 
the  same  distance  apart,  and  the  larger  the  circle 
the  greater  will  be  the  distance.  Now  let  a  beam 
of  red  light  fall  upon  the  flat  surface.  A  black 
spot  is  seen  at  the  centre ;  around  this  a  circle 
of  red  light,  then  a  dark  ring,  then  another  circle 
of  red  light,  and  so  alternating  to  the  circumfer- 
ence. By  careful  measurement  it  is  found  that 
the  distances  between  the  surfaces  of  the  glass 


20g  NATURAL  PHILOSOPHY. 

where  the  circles  of  red  light  appear,  are  as  the 
numbers  1,  2,  3,  &c.  This,  taken  in  connection 
with  what  we  know  already  of  the  theory  of  wave- 
motion,  suggests  at  once  the  cause.  There  are  two 
sets  of  waves,  one  reflected  from  the  upper  surface 
of  the  plane  glass,  and  the  other  from  the  lower 
surface  of  the  convex  glass.  Where  their  distance 
apart  is  less  than  a  wave-length,  they  interfere  and 
produce  darkness.  Where  it  is  1,  2,  3,  or  some 
whole-number  of  wave-lengths,  they  coalesce  and 
produce  a  wave  of  greater  intensity.  To  determine 
the  length  of  a  wave  of  red  light,  we  have  only  to 
measure  the  distance  between  the  two  glasses  at 
the  first  ring. 

When  beams  of  light  of  the  various  colors  are 
used,  circles  of  a  corresponding  color  are  obtained, 
and,  singularly  enough,  the  circles  are  of  dif- 
ferent diameters ;  red  light  gives  the  largest,  and 
violet  the  smallest.  We  hence  conclude  that  red 
waves  are  the  longest,  and  violet  the  shortest. 

Length  of  the  Waves. — The  minuteness  of  these 
waves  passes  comprehension.  40,000  red  waves 
and  60,000  violet  ones  are  comprised  within  a  single 
inch.  Knowing  that  light  moves  at  the  rate  of 
183,000  miles  per  second,  we  can  easily  calculate 
the  number  of  these  tiny  waves  which  reach  our 
eye  in  that  time.  When  we  look  at  a  red  object, 
414  million  million  of  ether  waves  break  on  the 
retina  every  moment,  and  with  a  violet  color  the 
number  reaches  666  million  million ! 


OPTICS.  209 

COLOR  is  exactly  analogous  to  pitch.  Violet  cor- 
responds to  the  high  and  red  to  the  low  sounds  in 
music.  Intensity  of  color,  like  that  of  sound,  de- 
pends on  the  amplitude  of  the  vibrations.  When  a 
body  absorbs  all  the  colors-  of  the  spectrum  except 
blue,  but  reflects  that  to  the  eye,  we  call  it  a  blue 
body ;  when  it  absorbs  all  but  green,  we  call  it  a 
green  body.  Red  glass  has  the  power  of  absorb- 
ing all  except  the  red  rays,  which  it  transmits. 
When  a  substance  reflects  all  the  colors  to  the  eye, 
it  seems  to  us  white.  If  it  absorbs  all  the  colors,  it 
is  black.  A  tint  is  produced  by  a  mingling  of  waves 
of  different  colors.  We  thus  see  that  color  is  not  an 
inherent  property  of  the  objects  around  us.  In  the 
darkness  all  bodies  are  devoid  of  color. 

The  play  of  colors  seen  in  mother-of-pearl  is  due 
to  the  interference  of  light  in  the  fine  grooves  caused 
by  the  edges  of  the  thin  overlapping  plates  of  which 
it  is  composed.  The  same  effect  may  be  seen  in  a 
putty  mould  of  the  pearl.  In  a  similar  manner  the 
plumage  of  certain  birds  reflects  changeable  hues. 
A  metallic  surface  ruled  with  fine  parallel  lines  not 
more  than  ^iVir  of  an  inch  apart,  gleams  with  bril- 
liant colors.  Thin  cracks  in  plates  of  glass  or  quartz, 
mica,  when  two  layers  are  slightly  separated — even 
the  scum  floating  in  stagnant  water,  break  up  the 
white  light  of  the  sunbeam  and  reflect  the  varying 
tints  of  the  rainbow.  The  rich  coloring  of  a  soap- 
bubble  is  given  it  by  the  film  of  water  which  runs 
from  the  top  down  the  sides,  and  thus  produces  in- 
terference of  light. 


210 


NATURAL  PHILOSOPHY. 


DIFFRACTION  OF  LIGHT  is  caused  by  a  beam  of  light 
passing  along  the  edge  of  some  opaque  body.  As 
the  waves  of  ether  strike  against  it,  they  put  in 
motion  another  set  of  waves  on  the  opposite  side 
which  interfere  with  the  first  system.  If  we  hold  a 
fine  needle  close  to  one  eye  and  look  toward  the 
window,  we  shall  see  several  needles.  Place  the 
blades  of  two  knives  closely  together  and  hold  them 
up  to  the  sky ;  a  most  beautiful  set  of  waving  lines 
of  interference  will  shade  the  open  space.  Most 
delicate  colors  are  seen  by  looking  at  the  sky  through 
the  meshes  of  a  veil,  or  at  a  lamp-light  through  a 
bird-feather  or  a  fine  slit  in  a  card. 

POLARIZED  LIGHT. — Double  Refraction. — If  we  could 
Fig.  155.       look  at  the  end  of  a  ray  of  light  as  we 
can  at  the  end  of  a  rod,  we  should  see 
the  particles  of  ether  swinging  swiftly 
to  and  fro,  crosswise,  in  the  direction  of 
all  the  diameters  (Fig.  155).     Certain 
crystals  have  the  power  of  sifting  and  arranging 
these  vibrations  into  two  sets  at  right  angles  to  each 

Fig.  156. 


other,  thus  making  a  ray  of  the  form  seen  in  Fig. 
156.  As  one  set  is  more  refracted  than  the  other  in 
passing  through  the  crystal,  the  ray  is  divided  into 
two  rays — the  ordinary  and  extraordinary.  Rays 


OPTICS. 


211 


Fig.  158, 


which  have  been  thus  sifted  constitute  polarized 
light.  Iceland  spar  (Fig.  157)  possesses  this  property 
of  double  refraction  in  a  re-  Fig.  157. 

markable  degree.  An  object 
iewed  through  it  in  any 
direction  not  parallel  to 
a  b  (the  optic  axis)  appears 
double.  If  the  crystal  is 
placed  on  a  dot  and  slowly  turned  around,  two  dots 
will  be  seen,  the  second  revolving  about  the  first. 
Light  may  be  po- 
larized by  reflection 
from  glass  at  a 
fixed  angle,  and  also 
by  passing  through  a 
thin  slice  of  tourma- 
line, —  a  transparent 
crystal  which  ab- 
sorbs the  ordinary 
and  transmits  the  extraordinary  rays.  Objects  ex- 
amined by  means  of  polarized  light  present  many 
curious  changes.  A  crystal  of  quartz  or  mica,  which 
appears  to  the  eye  like  common  glass,  reveals  a  se- 
ries of  beautifully  colored  rays,  due  to  the  inter- 
ference of  the  ordinary  and  extraordinary  rays  of 
light.  If  we  look  at  a  lamp-light  through  a  piece  of 
common  isinglass,  we  shall  see  a  beautiful  series  of 
polarized  rays  having  a  star-like  form.  The  angle 
of  the  rays  varies  with  different  kinds  of  mica.  If 
polarized  light  be  passed  through  common  glass  no 


Object  seen  through  Iceland  Spar. 


212  NA  TURAL  PHIL  OSOPHT. 

change  is  seen,  but  on  slight  pressure  a  system  of 
variegated  colors  appears.  This  method  of  exami- 
nation presents  a  most  delicate  means  of  determin- 
ing the  molecular  structure  of  a  body.  Some  sub- 
stances have  the  power  of  twisting  the  plane  of  the 
polarized  light.  Grape-sugar  turns  it  to  the  right, 
and  fruit-sugar  to  the  left.  The  French  Govern- 
ment use  a  polarizing  instrument,  in  which  this  prin- 
ciple is  applied  to  test  the  quality  of  the  sugar  im« 
ported  into  France. 

THE  RAINBOW  is  formed  by  the  refraction  and  re- 
flection of  the  sunbeam  in  drops  of  falling  water. 
The  white  light  of  the  sun  is  thus  decomposed  into 
its  simple  colors.  The  inner  arch  is  termed  the 
primary  bow ;  the  outer  or  fainter  arch,  the  second- 
ary. Each  of  these  contains  all  the  colors  of  the 
spectrum,  but  in  reverse  order.  The  rainbow  is  al- 
ways seen  in  the  quarter  of  the  heavens  opposite  to 
the  sun. 

Primary  Bow. — A  ray  of  light,  S",  enters,  and  is 
bent  downward  at  the  top  of  a  falling  drop,  passes 
to  the  opposite  side,  is  there  reflected,  then  passing 
out  of  the  lower  side,  is  bent  upward.  By  the  re- 
fraction the  ray  of  white  light  is  decomposed,  so 
that  when  it  emerges  it  is  spread  out  fan-like,  as  in 
the  solar  spectrum.  Suppose  that  the  eye  of  a  spec- 
tator is  in  a  proper  position  to  receive  the  red  ray, 
he  cannot  receive  any  other  color  from  the  same 
drop,  because  the  red  is  bent  upward  the  least,  and 
all  the  others  will  pass  directly-  over  his  head.  He 


OPTICS. 


213 


sees  the  violet  in  a  drop  below.    Intermediate  drops 
furnish  the  other  colors  of  the  spectrum. 


Fig.  159. 


Secondary  Bow. — A  ray  of  light,  S,  strikes  the 
bottom  of  a  drop,  v,  is  refracted  upward,  passes  to 
the  opposite  side,  where  it  is  twice  reflected,  and 
thence  passes  out  at  the  upper  side  of  the  drop. 
The  violet  ray  being  most  refracted,  is  bent  down 
to  the  eye  of  the  spectator.  Another  drop,  r,  re- 
fracting another  ray  of  light,  is  in  the  right  position 
to  send  the  red  ray  to  the  eye. 

Why  the  Boiv  is  circular. — In  the  primary  bow  it 
is  found  that  when  the  red  ray  leaves  the  drop,  it 
forms  an  angle  with  the  sun's  ray,  S  r,  of  about  42°, 
the  violet  40°.  These  angles  are  constant.  Let  a  b 
be  a  straight  line  drawn  from  the  sun  through  the 
'observer's  eye.  If  produced,  it  would  pass  through 


214 


NATURAL  PHILOSOPHY. 


the  centre  of  the  circle  of  which  the  rainbow  is  an 
arc.  This  line  is  termed  the  visual  axis.  It  is  par- 
allel to  the  rays  of  the  sun;  and  when  it  is  also 
parallel  to  the  horizon,  the  rainbow  is  an  exact  semi- 
circle. Suppose  the  line  E  v  in  the  primary  bow  to 
be  revolved  around  E  ft,  keeping  the  angle  b  E  v  un- 
changed ;  the  point  v  would  describe  a  circle  on  the 
sky,  and  every  drop  over  which  it  would  pass  would 
be  at  the  proper  angle  to  send  a  violet  ray  to  the  eye 
at  E.  Imagine  the  same  with  the  drop  r.  We  can 
thus  see  (1)  that  the  bow  must  be  circular ;  (2)  that 
when  the  sun  is  high  in  the  heavens,  the  whole  bow 
sinks  below  the  horizon ;  (3)  that  the  lower  the  sun 
che  larger  is  the  visible  circumference  ;  and  (4)  that 
on  lofty  mountains  a  perfect  circle  may  sometimes 
be  seen. 

Halos,  coronas,  sundogs,  circles  about  the  moon, 
the  gorgeous  tinting  at  sunrise  and  sunset,  are 
all  produced  by  the  refraction  and  reflection  of  the 
sun's  rays  when  passing  through  the  clouds  in  the 
upper  regions  of  the  atmosphere.  The  phenom- 
enon familiarly  known  as  the  "  sun's  drawing  wa- 
ter," consists  merely  of  the  long  shadows  of  broken 
clouds.* 

SPHERICAL  ABERRATION. — Bays  which  pass  through 
a  lens  near  the  edge  are  brought  to  a  focus  sooner 
ihan  those  nearer  the  centre.  Therefore,  when  an 

*  Spectrum  analysis,  twilight,  and  other  kindred  topics,  are 
best  understood  in  their  relations  to  Astronomy.  See  "  Fourteen 
Weeks  in  Astronomy." 


OPTICS.  2  j  5 

image  is  clear  around  the  edge,  it  will  be  indistinct 
at  the  centre,  and  vice  versa.  This  wandering  of  the 
rays  from  the  focus  is  termed  spherical  aberration. 

Chromatic  aberration  is  caused  by  the  different  re- 
frangibility  of  the   several   colors  which   compose 
white  light.     The  violet,  being  bent  most,  tends  to 
come  to  a  focus  sooner  than  the  red,  which  is  bent 
least.     This  causes  the  play  of  colors  seen  around 
the  image  produced  by  an  ordinary  glass.     It  is 
remedied  by  using    a    second 
lens     of     different     dispersive 
power,   which    counteracts  the 
effect  of  the  first.      (Fig.  160.) 
Such  a  lens  is  said  to  be  achromatic  (colorless). 

OPTICAL  INSTRUMENTS. 

MICROSCOPES  (to  see  small  things)  are  of  two  kinds, 
simple  and  compound.  The  former  consists  of  a 
double  convex  lens ;  the  latter  contains  at  least 
two  lenses. 

At  M  is  a  mirror  which  reflects  the  rays  of 
light  through  the  object  a.  The  object-lens  o 
forms,  in  the  tube  above,  a  magnified  inverted 
image  of  the  object.  The  eye-lens  O  magnifies  this 
image.  If  a  microscope  increases  the  diameter  of 
an  object  100  times,  it  is  said  to  have  a  power  of 
100  diameters.  In  that  case  the  surface  is  magnified 
1002= 10,000  times.  To  prevent  spherical  aberra- 
tion the  object-lens  is  ma<le  very  small. 


2i6  NATURAL  PHILOSOPHY. 

Fig.  161. 


A  Compound  Microscope. 

TELESCOPES  (to  see  afar  off)  are  of  two  kinds,  re« 
fleeting  and  refracting.  The  former  contains  a  large 
metallic  mirror  (speculum)  which  reflects  the  rays 
of  light  to  a  focus.  The  observer  stands  at  the 
side  and  examines  the  image  with  an  eye-piece. 


21 


A  Reflecting  Telescope. 

The  largest  reflecting  telescope  ever  made  is  that  of 
Lord  Eosse.  Its  speculum  has  a  diameter  of  6  feet, 
and  a  focal  distance  of  53  feet.  (See  frontispiece  of 
Astronomy.) 

The  refracting  telescope,  like  the  microscope,  con- 
10 


2  j  g  NA  TUX  A  L  PHIL  OSOPIIY. 

tains  an  object-lens  o  which  forms  an  image  a  b. 
This  is  viewed  by  means  of  the  eye-piece  O,  which  pro- 
duces a  magnified  and  inverted  image  cd.  The  objects 
seen  in  the  heavens  are  so  far  distant  that  the  rays 
of  light  are  nearly  parallel,  and  hence  there  is  little 
spherical  aberration.  The  object-lens  may  there- 

Pig.  163. 


fore  be  made  of  any  size  without  rendering  the  im- 
age indistinct.  The  larger  this  lens,  the  more  light 
is  collected  with  which  to  view  the  image.  The 
magnifying  power  is  principally  due  to  the  eye- 
piece. The  great  telescope  in  the  observatory  at 
Chicago  is  the  best  in  the  world.  The  diameter  of 
its  object-glass  is  18J  inches — equivalent  to  enlarg- 
ing the  pupil  of  the  astronomer's  eye  to  that  size. 
It  was  made  by  Alvan  Clark  &  Sons,  of  Cambridge, 
Massachusetts. 

The  inversion  of  the  object  is  of  no  practical  im- 
portance for  astronomical  purposes.  For  terrestrial 
observations  additional  lenses  are  used  to  invert  the 
image. 

The  opera-glass  contains  an  object-glass  O  and  an 
eye-piece  o.  The  latter  is  a  double  concave  lens ; 
this  increases  the  visual  angle  by  diverging  the  rays 


2I9 


Pig.  165. 


of  light,  which  would  otherwise  come  to  a  focus  be* 
yond  the  eye-piece.  An  erect  and  magnified  image 
is  seen  at  a  b. 

The  stereoscope  contains  por- 
tions of  two  convex  lenses,  as 
shown  in  Fig.  165.  Two  photo- 
graphs A  and  B  are  taken  by  two 
cameras  which  are  slightly  in- 
clined to  each  other.  This  pro- 
duces two  pictures  precisely 
like  the  two  views  we  always 
obtain  of  an  object  by  the  use 
of  both  eyes.  The  blending  of 
the  two  at  C  causes  the  appear- 
ance of  solidity. 

The  magic  lantern,  or  stereop- 
ticon,  contains  a  reflector  M, 
which  condenses  the  rays  of  a  powerful  oil  or  cal- 
cium light  upon  a  lens  L.  They  are  here  converged 
upon  the  object  ab.  Thence  a  double  lens  m  throws 
a  highly  magnified  image  on  the  screen  A  B.  Dis- 
solving views  are  produced  by  the  use  of  two  lanterns 
which  contain  the  separate  scenes  which  are  to  melt 
into  each  other. 


220 


NATURAL  PHILOSOPHY. 
Fig.  166. 


The  Camera  used  by  photographers  contains  a 
double-convex  lens  at  A,  which  throws  an  inverted 


Fig.  167. 


image  of  the  object  upon  the  ground-glass  screen 
EB. 

THE  EYE  is  the  most  perfect  optical  instrument. 
It  is  rarely,  if  ever,  troubled  by  spherical  or  chro- 
matic aberration,  and  is  self-focusing.  It  closely 
resembles  a  camera.  The  outer  membrane  of  the 
eye  is  termed  the  Sclerotic  coat,  S.  It  is  tough. 


OPTICS.  221 

white,  opaque,  and  firm.  A  little  portion  in  front, 
termed  the  Cornea,  c,  is  transparent ;  this  is  convex, 
and  is  set  into  the  sclerotic  coat  somewhat  like  a 
watch-crystal.  The  middle  or  Choroid  coat,  C,  is  soft 

Pig.  168. 


and  delicate,  like  velvet.  B  lines  the  inner  part  of 
the  eye.  It  is  covered  with  a  black  pigment,  which 
absorbs  the  superfluous  light.  Upon  it  the  optic 
nerve,  which  enters  at  the  rear,  expands  in  a  net- 
work of  delicate  fibres  termed  the  Retina.  This 
is  the  seat  of  vision.  Back  of  the  cornea  is  a  trans- 
parent limpid  fluid,  the  aqueous  humor.  The  anterior 
chamber,  filled  with  this  liquid,  is  closed  at  the  back 
by  a  colored  curtain,  h  i,  the  Iris.  The  Pupil  is  a 
round  hole  in  the  Iris.  The  Crystalline  lens,  o,  sep- 
arates the  two  chambers  of  the  eye.  It  is  a  double- 
convex  lens,  tough  as  gristle  and  transparent  as 
glass.  It  is  composed  of  concentric  layers,  like  an 
onion.  The  posterior  chamber  is  filled  with  the 
vitreous  humor,  which  is  a  transparent,  jelly-like 
liquid,  resembling  the  white  of  an  egg. 

Let  A  B  represent  an  object  in  front  of  the  eye. 


222  NA  TURA  L  PHIL  OSOPHY. 

Bays  of  light  are,  first  refracted  by  the  aqueous 
humor,  then  falling  upon  the  crystalline  lens  they 
are  further  refracted,  and  lastly  are  refracted  by  the 
vitreous  humor  and  form  an  image  a  b  on  the  retina. 
This  is  smaller  than  the  object,  and  inverted. 

The  more  distant  the  object,  the  smaller  the  pic- 
ture. To  render  vision  distinct,  the  rays  must  be 
accurately  focused  on  the  retina.  If  we  gaze  stead- 
ily at  an  object  near  by,  and  then  suddenly  observe 
some  distant  one,  we  find  our  vision  blurred.  In  a 
few  moments  it  becomes  clear  again.  This  shows 
that  the  eye  has  the  power  of  adapting  itself  to  the 
varying  distances  of  objects,  which  is  done  by  a  vari- 
ation in  the  convexity  of  the  crystalline  lens. 

When  a  body  is  held  very  near  the  eye,  the  lens 
has  not  sufficient  power  to  converge  the  rays  upon 
the  retina  in  a  distinct  image.  When  the  distance 
at  which  a  clear  vision  takes  place  is  less  than  four 
or  five  inches,  the  person  is  said  to  be  near-sighted, 
and  when  greater  than  ten  or  twelve,  to  be  far- 
sighted.  This  difference  lies  in  the  shape  of  the  eye- 
ball. In  far-sightedness  the  ball  is  too  flat,  and  the 
retina  is  too  near  the  lens  ;  in  near-sightedness  the 
ball  is  elongated,  so  that  the  retina  is  too  far  bact; 
from  the  lens.  The  former  can  be  remedied  by  con- 
vex glasses,  which  bring  the  rays  to  a  focus  sooner, 
and  the  latter  by  concave  glasses,  which  throw  the 
focus  further  back.  In  old  age  the  eye  loses  the 
power  of  adjusting  the  crystalline  lens ;  elderly 
people,  therefore,  hold  a  book  at  some  distance 


OPTICS.  223 

from  the  eye.  They  are  aided  by  using  convex 
glasses. 

The  retina  retains  any  impression  made  upon  it 
for  about  one-eighth  of  a  second.  This  explains 
why  a  wheel,  when  rapidly  revolved,  appears  solid, 
or  a  lighted  brand  like  a  ring  of  fire.  On  the  other 
hand,  it  requires  a  moment  for  an  impression  to  be 
made.  Thus  a  wheel  may  be  whirled  so  swiftly  that 
its  spokes  become  invisible. 

Some  eyes  are  entirely  uninfluenced  by  certain 
colors,  as  some  ears  are  deaf  to  certain  sounds. 
Color-blindness  is  most  commonly  noticed  in  refer- 
ence to  red,  green,  and  blue.  Doubtless  railway  ac- 
cidents have  often  occurred  through  this  natural  in- 
ability to  distinguish  signals.  Dr.  Mitchell  mentions 
the  case  of  a  naval  officer  who  chose  for  his  uni- 
form a  blue  coat  and  red  waistcoat,  fully  believing 
them  to  be  of  the  same  color.  He  also  tells  of  a 
tailor  who  mended  a  black  silk  waistcoat  with  a 
piece  of  crimson,  and  of  another  who  put  a  red  col- 
lar on  a  blue  coat.  Dalton  could  only  see  two  colors, 
blue  and  yellow,  in  the  solar  spectrum,  and  having 
once  dropped  a  piece  of  red  sealing-wax  in  the 
grass,  he  could  not  find  it  by  the  difference  in  color. 
The  range  of  the  eye  is  much  less  than  that  of  the 
ear.  The  latter  is  about  eleven  octaves,  while  the 
former  never  exceeds  a  single  octave. 

The  diameter  of  the  eye  is  less  than  an  inch ; 
yet,  as  we  look  over  an  extended  landscape,  every 
feature,  with  all  its  variety  of  shade  and  color, 


224  NATURAL   PHILOSOPHY. 

is  repeated  in  miniature  on  the  retina.  Millions 
upon  millions  of  ether  waves,  converging  from  every 
direction,  break  on  that  tiny  beach,  while  we,  obliv- 
ious to  all  the  marvellous  nature  of  the  act,  think 
only  of  the  beauty  of  the  revelation. 

^Practical  Questions.— \.  Why  is  the  secondary  bow  fainter  than  the 
primary?  Why  are  the  colors  reversed?  2.  Why  can  we  not  see  around  the 
corner  of  a  house,  or  through  a  bent  tube  ?  3.  What  color  would  a  painter 
use  if  he  wished  to  represent  an  opening  into  a  dark  cellar?  4.  Is  white  a 
color?  Is  black?  5.  By  holding  an  object  nearer  a  light,  will  it  increase  or 
diminish  the  size  of  the  shadow  ?  6.  What  must  be  the  size  of  a  glass  in  or- 
der to  reflect  a  full-length  image  of  a  person  ?  Ans.  Half  the  person's  height. 
7.  Where  may  we  see  a  rainbow  in  the  morning  ?  8.  Can  two  spectators  see 
the  same  bow  ?  9.  Why,  when  the  drops  of  water  are  falling  through  the  air, 
does  the  rainbow  appear  stationary?  1O.  Why  can  a  cat  see  in  the  night? 
11.  Why  cannot  an  owl  see  in  daylight?  12.  Why  are  we  blinded  when  we 
pass  quickly  from  a  dark  into  a  brilliantly  lighted  room  ?  1 3 .  If  the  light  of 
the  sun  upon  a  distant  planet  is  only  Vioo  of  that  which  we  receive,  how  does 
its  distance  from  the  sun  compare  with  ours  ?  1 4.  If  when  I  sit  six  feet  from 
a  candle  I  receive  a  certain  amount  of  light,  how  much  shall  I  diminish  it  if  I 
move  back  six  feet  further?  15.  Why  do  drops  of  rain,  in  falling,  appear  like 
liquid  threads?  16.  Why  does  a  towel  turn  darker  when  wet?  17.  Does 
color  exist  in  the  object,  or  in  the  mind  of  the  observer  ?  18.  Why  is  lather 
opaque,  while  air  and  a  solution  of  soap  are  each  transparent?  19.  Why 
does  it  whiten  molasses  candy  to  "pull  it?  "  2O.  Why  does  plastering  be- 
come lighter  in  color  as  it  dries?  21.  Why  does  the  photographer  use  a 
kerosene  oil  lamp  in  the  "dark  room?"  22.  Is  the  common  division  of 
colors  into  "cold"  and  "warm"  verified  in  philosophy?  23.  Why  is  the 
image  on  the  camera,  Fig.  167,  inverted?  24.  Why  is  the  second  image  seen 
in  a  mirror,  Fig.  134.  brighter  than  the  first?  25.  Why  does  a  blow  on  the 
head  make  one  "  see  stars  ?  "  Ans.  The  blow  excites  the  optic  nerve,  and  so 
produces  the  sensation  of  light,  26.  What  is  the  principle  of  the  kaleido- 
scope ?  Ans.  It  contains  three  mirrors  set  at  an  angle  of  60°.  Small  bits  of 
colored  glass  at  one  end  reflect  to  the  eye,  at  the  other,  multiple  images  which 
change  in  varying  patterns  as  the  tube  is  revolved.  27.  Which  can  be  heard 
at  the  greater  distance,  noise  or  music  ? 


HEAT. 

DEFINITIONS. — Luminous  heat  is  that  which  radi- 
ates from  a  luminous  body.  Ex.  :  An  iron  heated 
to  whiteness.  Obscure  heat  is  that  from  a  non-lumi- 
nous source.  A  diathermanous  (dia,  through,  and 
thermos,  warm)  body  is  one  which  allows  the  heat  to 
pass  through  it  readily.  Rock-salt  is  the  most  per- 
fect diathermanous  solid  known.  It  is  to  heat  what 
glass  is  to  light.  Cold  is  a  merely  relative  term,  in- 
dicating the  absence  of  heat  in  a  greater  or  less  de- 
gree. Gases  and  Vapors  differ  but  slightly.  The 
former  retain  their  form  at  all  ordinary  tempera- 
tures ;  Ex. :  Air.  The  latter  are  readily  condensed  ; 
Ex.  :  Steam. 

THE  INTIMATE  RELATION  BETWEEN  LlGHT  AND  HEAT. 

— Thrust  a  cold  iron  into  the  fire.  It  is  at  first  dark, 
but  soon  becomes  luminous,  like  the  glowing  coals, 
liaise  the  temperature  of  a  platinum  wire.  We  soon 
feel  the  radiation  of  obscure  heat-rays.  As  the 
metal  begins  to  glow,  our  eye  detects  a  red  color, 
then  orange  combined  with  it,  then  green,  and  so 
on  through  the  scale  of  the  spectrum.  At  last  all 


228  NA  TUMAL  PHIL  080PHY.  *. 

the  colors  are  emitted,  and  the  metal  is  dazzling 
white.  All  bodies  become  luminous  at  a  fixed  tem- 
perature. Heat  may  be  reflected,  refracted,  and 
even  polarized.  It  radiates  in  straight  lines  equally 
in  every  direction,  and  decreases  in  intensity  as  the 
square  of  the  distance.  It  moves  with  the  same 
velocity  as  light.  Heat  and  light  come  to  us  com- 
bined in  the  sunbeam.  It  is  therefore  believed  that 
they  are  the  same — that  light  is  only  luminous  heat, 
and  that  the  three  classes  of  waves  in  the  solar 
spectrum  differ,  as  one  color  differs  from  another, 
in  the  rapidity  of  the  vibrations.  The  longer  and 
slower  waves  of  ether  fall  upon  the  nerves  of  touch, 
and  produce  the  sensation  of  heat.*  The  more  rapid 
ones  are  peculiarly  adapted  to  affect  the  optic  nerve 
and  produce  the  sensation  of  light.  The  shortest 
and  quickest  cause  chemical  changes. 

THEORY  OF  HEAT. — Heat  is  only  a  mode  of  motion. 
Best  is  unknown  in  nature.     Even  the  molecules  of 


*  It  is  now  believed  tliat  the  particles  of  the  nerves  vibrate, 
and  thus  communicate  to  the  brain  the  impressions  made  by  ex- 
ternal objects.  Each  of  the  five  classes  of  nerves  seems  to  be 
adapted  to  transmit  vibrations  of  its  own  kind,  while  it  is  insen- 
sible to  all  others.  Thus,  if  the  rate  of  oscillation  be  less  than 
that  of  red,  or  more  than  that  of  violet,  the  optic  nerve  is  unin- 
fluenced by  the  waves.  We  cannot  see  with  our  fingers,  nor 
taste  with  our  ears.  Nerves  transmit  motion  at  the  rate  of  about 
93  feet  per  second.  If,  then,  a  man,  six  feet  high,  were  to  step  on 
a  nail,  it  would  require  nearly  an  eighth  of  a  second  for  the  in- 
formation to  be  carried  by  the  sensor  nerve  to  the  brain,  and  for 
the  order  to  lift  the  foot  to  be  returned  by  the  motor  nerve  to  th« 
suffering  member. 


HE  A  T.  220 

a  solid  are  in  constant  vibration.  As  the  worlds  in 
space  are  revolving  about  each  other  in  inconceiv- 
ably vast  orbits,  so  each  body  forms  a  miniature 
system,  its  molecules  revolving  in  inconceivably 
minute  orbits.  When  we  increase  the  rapidity  of  this 
motion,  we  heat  the  body ;  when  we  decrease  it,  we 
cool  the  body.  The  vacant  spaces  between  the 
molecules  are  filled  with  ether.  As  the  air  moving 
among  the  limbs  of  a  tree  sets  its  boughs  in  motion, 
and  in  turn  may  be  kept  in  motion  by  the  waving 
branches,  so  this  ether  may  put  the  molecules  in 
vibration,  or  be  thrown  into  motion  by  them.  Ex. : 
Insert  one  end  of  a  poker  into  the  fire.  The  parti- 
cles in  contact  with  the  heat  are  made  to  vibrate 
intensely ;  these  swinging  atoms  strike  their  neigh- 
bors, and  so  on,  atom  by  atom,  until  the  oscillation 
reaches  the  other  end.  If,  now,  we  handle  the  poker, 
the  motion  is  imparted  to  the  delicate  nerves  of 
touch  ;  they  carry  it  to  the  brain,  and  pain  is  felt. 
In  popular  language,  "  the  iron  is  hot,"  and  we  are 
burned.  If,  without  touching  it,  we  hold  our  hand 
near  the  poker,  the  ether  waves  set  in  motion  by  the 
whirling  atoms  of  iron  strike  against  the  hand,  and 
produce  a  less  intense  sensation  of  heat.  In  the 
former  case,  the  fierce  motion  is  imparted  directly ; 
in  the  latter,  the  ether  acts  as  a  carrier  to  bring  it 
to  us. 

QUALITY  OF  HEAT. — As  some  sounds  are  shrill  and 
piercing,  others  deep  and  heavy,  so  some  kinds  of 
heat  are  keen  and  penetrating,  others  mild  and  dif- 


22O  NATURAL  PHILOSOPHY. 

fusive.  This  difference  depends  on  the  length  of  the 
waves  and  their  combined  rates  of  vibration.  Thus 
the  chirp  of  the  cricket  compares  with  the  heat  of  a 
glowing  furnace,  and  the  soft  tones  of  an  organ  with 
the  genial  radiation  of  a  steam-pipe.  Pitch  in  mu- 
sic, variety  in  color,  and  degrees  in  heat,  are  there- 
fore intimately  related. 

THE  SOURCES  OF  HEAT  are  the  sun,  stars,  mechan- 
ical and  chemical  forces. 

(1.)  The  molecules  of  the  sun  and  stars  are  in 
rapid  vibration.  These  set  in  motion  waves  of 
ether,  which  dart  with  the  velocity  of  light  across 
the  intervening  space,  and  surging  against  the 
earth,  give  up  their  motion  to  it.  (2.)  Friction 
and  percussion  produce  heat,  because  additional 
motion  is  thereby  imparted  to  the  vibrating  par- 
ticles. Savages  obtain  fire  by  rubbing  together 
two  pieces  of  wood.  A  horse  hits  his  steel  shoes 
against  a  stone  and  "  strikes  fire ;"  little  particles  of 
the  metal  torn  off  are  heated  by  the  shock,  so  that 
they  burn  as  sparks.  The  bearings  of  machinery 
become  hot,  unless  the  friction  is  diminished  by 
grease.  A  train  of  cars  is  stopped  by  the  pressure 
of  the  brakes.  If  we  watch  them  in  a  dark  night, 
we  shall  see  the  sparks  flying  from  the  wheels,  the 
motion  of  the  train  being  converted  into  heat.  A 
blacksmith  pounds  a  piece  of  iron  until  it  glows. 
The  force  of  his  strokes  sets  the  particles  of  the 
metal  vibrating  rapidly  enough  to  send  ether  waves 
of  such  swiftness-  as  to  affect  the  eye  of  the  ob- 


HEAT.  2$I 

server.*  A  piece  of  wood  may  be  heated  by  simply 
squeezing  it  in  a  hydraulic  press.  At  the  exposition 
in  Paris,  chocolate  was  kept  hot  by  means  of  two 
copper  plates  which  were  rubbed  together  by  ma- 
chinery. As  a  cannon-shot  strikes  an  iron  target,  a 
sheet  of  flame  pours  from  it.  Our  earth  moves  with 
a  velocity  of  over  68,000  miles  per  hour.  Were  it 
instantly  stopped,  enough  heat  would  be  produced 
to  change  the  entire  globe  to  vapor.  (3.)  Chemical 
action  is  most  commonly  seen  in  fire.  The  oxygen 
of  the  air  has  an  affinity  for  the  carbon  and  hydro- 
gen of  the  fuel.  They  rush  together.  As  they  strike, 
their  motion  is  stopped.  The  shock  sets  the  mole- 
cules in  vibration.  They  impart  their  motion  to  the 
ether,  and  thus  start  waves  of  heat. 

THE  MECHANICAL  EQUIVALENT  OP  HEAT. — In  these 
various  changes  of  mechanical-motion  into  heat- 
motion  no  force  has  been  lost.  The  blacksmith's 
hammer  falling  on  the  anvil  gives  rise  to  a  definite 
amount  of  heat.  If  the  heat  could  be  gathered  up, 
it  would  be  sufficient  to  lift  the  hammer  to  the  point 
from  which  it  fell.  No  force  can  be  annihilated. 
If  destroyed  in  one  form,  it  reappears  in  another 
without  loss.  A  pound-weight  falling  from  a  height 
of  772  feet,  would  generate  enough  heat  to  raise  the 


*  Text-books  frequently  assert  that  "  iron,  once  treated  in  this 
way,  cannot  again  be  made  red-hot  by  hammering,  unless  subse- 
quently reheated  in  the  forge."  Even  Miller  gives  currency  to 
this  statement.  Any  blacksmith  will  convince  you  of  its  utter 
falsity  by  actual  experiment. 


222  NATURAL  PHIL OSOPH7. 

temperature  of  one  pound  of  water  one  degree;  con- 
versely, the  amount  of  heat  necessary  to  elevate 
a  pound  of  water  one  degree,  would  raise  a  pound- 
weight  to  the  height  of  772  feet.  This  is  called 
"  Joule's  latv"  or  the  "  mechanical  equivalent  of  heat." 

CHANGE  or  STATE  BY  HEAT. 

LATENT,  SENSIBLE,  AND  SPECIFIC  HEAT. — When  a 
body  is  heated,  the  heat-force  is  divided  into  two 
parts :  one  portion  elevates  the  temperature,  and 
the  other  increases  the  size.  The  former  can  be 
detected  by  the  touch,  and  is  called  sensible  heat. 
The  latter  tends  to  counteract  the  force  of  Cohesion, 
and  is  neutralized  so  that  it  cannot  be  detected  by 
the  touch ;  it  is  therefore  termed  latent  heat.  Sub- 
stances vary  in  their  application  of  the  heat-force. 
Some  devote  more  to  temperature,  others  to  expan- 
sion. Therefore,  if  the  same  amount  of  heat  be  ap- 
plied to  the  same  bulk  of  different  substances,  they 
will  not  indicate  the  same  temperature ;  and  on  the 
other  hand,  when  various  bodies  indicate  the  same 
temperature  by  a  thermometer,  they  may  possess 
vastly  different  quantities  of  heat.  Steam  contains 
the  greatest  amount  of  latent  heat  of  any  known  sub- 
stance, except  hydrogen,  yet  it  indicates  no  higher 
temperature  than  boiling  water.  The  relation  be- 
tween sensible  and  latent  heat  is  termed  specific 
heat.  It  is  the  quantity  of  heat  that  is  required  to 
raise  a  given  weight  of  any  substance  1°  in  tempera- 
ture, compared  with  the  quantity  required  to  elevate 


HEAT.  233 

the  same  weight  of  water  1°.  Thus,  a  quantity  of 
heat  which  elevates  the  temperature  of  a  pound  of 
water  1°  would  raise  that  of  a  pound  of  mercury  30°. 
Hence,  taking  water  as  the  standard,  the  specific 
heat  of  mercury  is  only  .033. 

Latent  heat  is  not  lost. — In  the  various  changes  of 
state  of  which  we  shall  now  speak,  wherein  bodies 
pass  from  solid  to  liquid  and  from  liquid  to  the 
gaseous  form,  sensible  heat  becomes  latent.  Thus, 
one  who  has  melted  snow,  or  "  boiled  away"  water, 
knows  how  slow  is  the  process,  and  how  much  heat 
is  consumed.  When  the  vapor  or  liquid  passes  back 
into  its  original  state,  the  latent  heat  is  restored 
again  as  sensible  heat.  The  following  curious  para- 
dox illustrates  this  thought :  Freezing  is  a  warming 
process,  and  thawing  a  cooling  process. 

Freezing -mixtures  depend  on  the  principle  of  latent 
heat.  Their  most  common  use  is  in  freezing  ice- 
cream. Salt  and  powdered  ice  are  employed.  Salt 
has  a  great  attraction  for  water.  It  therefore  dis- 
solves the  ice  to  get  it,  and  then  itself  dissolves  in 
the  water  thus  obtained.  In  this  process  two  solids 
pass  into  the  liquid  form.  The  heat  necessary  for 
this  change  of  state  is  absorbed  mainly  from  the 
cream. 

I.  EXPANSION. — By  the  addition  of  heat  the  mole- 
cules are  urged  into  swifter  motion,  and  therefore 
pushed  further  apart,  increasing  the  size  of  the  body. 
Hence  the  law,  "Heat  expands  and  cold  contracts." 

(1.)  Solids  expand  uniformly;  i.  e.,  a  definite  rise 


234  NATURAL  PHILOSOPHY. 

in  temperature  produces  a  fixed  increase  of  size. 
Different  substances,  however,  expand  unequally. 
Zinc  dilates  more  than  iron,  and  iron  more  than 
glass.  The  force  of  the  expansion  is  irresistible. 
It  is  said  that  iron,  heated  from  zero  up  to  the  boil- 
ing point  of  water,  exerts  a  pressure  equal  to  16,000 
times  that  of  gravity.  When  it  cools,  it  contracts 
with  the  same  force.  Practical  applications  of  this 
principle  abound  in  the  arts.  A  carriage-tire  is  put 
on  when  hot,  that,  when  cooled,  it  may  bind  the 
wheel  together.  Rivets  used  in  fastening  the  plates 
of  steam-boilers  are  inserted  red-hot.  "  The  pon- 
derous iron  tubes  of  the  Britannia  bridge  writhe  and 
twist,  like  a  huge  serpent,  under  the  varying  influence 
of  the  solar  heat.  A  span  of  the  tube  is  depressed 
but  a  quarter  of  an  inch  by  the  heaviest  train  of 
cars,  while  the  sun  lifts  it  2|  inches."  The  Bunker- 
hill  monument  nods  as  it  follows  the  sun  in  its  daily 
course.  Lead  and  zinc,  on  cooling,  do  not  contract 
to  their  original  dimensions,  but  their  particles 
slide  over  each  other  in  expanding ;  in  this  man- 
ner the  linings  of  sinks  become  puckered.  On  the 
other  hand,  this  force  of  expansion  must  be  guarded 
against.  In  laying  long  water-pipes,  some  of  the 
tubes  are  made  to  slip  into  each  other  with  tele- 
scopic slides.  Tumblers  of  thick  glass  often  break 
on  the  sudden  application  of  heat,  because  the  sur- 
face dilates  before  the  motion  has  time  to  reach 
the  interior.  Draughts  of  cold  air  frequently  crack 
heated  lamp-chimneys,  for  a  similar  reason.  (2). 


HEAT.  23$ 

liquids  are  much  more  sensitive  to  heat  than  solids, 
but  do  not  expand  as  equally.  (3).  Gases  expand 
uniformly  ^-g-  of  their  bulk.  490  cubic  inches  of 
any  gas,  at  32°  F.,  if  heated  1°,  become  491  cubic 
inches. 

The  Mercurial  Thermometer  is  an  instrument  for 
measuring  the  temperature  by  the  expansion  of 
mercury.  As  this  metal  freezes  at  —39°  F.,  colored 
alcohol  is  used  for  low  temperatures.  The  method 
of  filling  a  thermometer  may  be  illustrated  in  the 
following  manner.  Take  the  glass  tube  shown  in 
Fig.  67,  and  hold  the  bulb  in  the  flame  of  an  alcohol 
lamp  until  the  air  is  nearly  expelled.  Then  plunge 
the  stem  in  some  colored  water.  As  soon  as  the 
bulb  cools,  the  water  will  rise  and  partly  fill  it. 
Heat  the  bulb  again  in  the  flame  until  the  steam 
pours  out  of  the  stem.  On  inserting  it  a  second 
time,  the  water  will  entirely  fill  the  bulb.  In  the 
manufacture  of  thermometers,  it  is  customary  to  have 
a  cup  blown  at  the  upper  end  of  the  stem.  This 
is  filled  with  mercury,  and  the  air,  when  expand- 
ed, bubbles  out  through  it,  while  the  metal  trickles 
down  as  the  bulb  cools.  The  mercury  is  now  heated 
to  as  high  a  temperature  as  the  thermometer  is  in- 
tended to  measure,  when  the  tube  is  melted  off  and 
sealed  at  the  extremity  of  the  column  of  mercury. 
The  metal  contracts  on  cooling,  and  leaves  a  vacuum 
above.  Each  thermometer  is  graduated  separately. 
It  is  put  in  melting  ice,  and  the  point  to  which  the 
mercury  sinks  is  marked  32° — Freezing-paint.  It  is 


336  NA  TURAL  PHIL  OSOPHY. 

then  placed  in  a  steam-bath,  and  the  point  to  which  the 
mercury  rises  (when  the  barometric  column  stands  at 
30  in.)  is  marked  212° — Boiling-point.  This  constitutes 
K    169       what  is   called  Fahrenheit's   scale    (F.) 
It  is  said  that  the  inventor  placed  the 
zero-point  32°  below  the  temperature  of 
freezing  water,  because  he  thought  that 
to  be  absolute  cold.     In  the  Centigrade 
scale  (C.),  the  freezing-point  is  marked  0, 
and  the  boiling-point  100.    1°  C.  =  1.8°F. 
In  Reaumur's  scale  (R.),  the  boiling-point 
is  fixed  at  80°. 

II.  LIQUEFACTION. — When  heat  is  add- 
ed to  a  solid  body,  the  temperature  rises 
until  the  freezing-point  (melting-point)  is 
reached,  when  it  becomes  stationary. 
The  force  is  now  all  applied  to  neutral- 
izing the  Cohesive  attraction.  The  expan- 
sion continues.  The  molecules  are  pushed  further 
and  further  apart,  until,  escaping  the  grasp  of  Cohe- 
sion, they  move  freely  on  each,  other.  This  constitutes 
liquefaction,  as  seen  in  the  melting  of  ice,  iron,  &c.  In 
this  process  large  quantities  of  heat  become  latent  in 
the  body.  If  ice  at  32°  be  melted,  142°  of  heat  will 
disappear,  and  the  water  will  be  at  only  32°.  Hence, 
to  convert  1  Ib.  of  ice  at  32°  into  water  at  32°, 
enough  heat  must  be  used  to  raise  142  Ibs.  of  water 
from  32°  to  33°. 

Liquefaction  of  gases. — When  a  gas  is  cooled  the 
repellant  force  is  weakened,  and  the  molecules  once 


HEAT. 


237 


Approach  each  other.  By  the  combined  action 
of  cold  and  pressure  the  particles  of  almost  every 
known  gas  have  been  brought  near  enough  for  the  at- 
traction of  Cohesion  to  grasp  them.  When  the  pres- 
sure is  removed,  the  gaseous  form  is  quickly  resumed. 
III.  YAPOKIZATION. — If  heat  be  applied  to  a  liquid, 
the  temperature  rises  until  the  boiling-point  is  reached, 
when  it  stops.  The  expansion,  however,  continues 
until  the  motion  is  so  violent  as  to  overcome  the  Co- 
hesive force  and  to  throw  off  particles  of  the  liquid. 

Fig.  170. 


The  vapor  thus  formed  does  not  contain  any  solid 
which  may  be  dissolved  in  the  liquid.  This  princi- 
ple is  applied  to  distillation.  Ex.  :  Pure  or  distilled 
water  is  obtained  by  heating  it  in  a  boiler  A,  whence 


238 


NATURAL  PHILOSOPHY. 


the  steain  passes  through  the  pipe  C  and  the  worm 
within  the  condenser  S,  where  it  is  condensed  and 
drops  out  into  the  vessel  D.  The  pipe  is  coiled  in  a 
spiral  form  within  the  condenser,  and  is  hence  termed 
the  worm.  The  condenser  is  kept  full  of  cold  water 
by  means  of  the  tub  at  the  left.  By  careful  regula- 
tion of  the  heat,  one  liquid  may  be  separated  from 
another  by  distillation.  (See  Chemistry,  p.  196.) 

The  boiling-point. — When  we  heat  water,  the  bub- 
bles which  pass  off  first  contain  merely  the  air  dis- 
solved in  the  liquid  ;  next  bubbles  of  steam  form  on 
the  bottom  and  sides  of  the  vessel,  and,  rising  a 
little  distance,  are  crushed  in  by  the  cold  water  and 
condensed.  In  breaking  they  produce  that  peculiar 
sound  known  as  "  simmering."  They  ascend  higher 
and  higher  as  the  temperature  of  the  water  rises, 
until  at  last  they  break  at  the  surface,  and  the  steam 
passes  off  into  the  air.  The  violent  agitation  of  the 
water  produced  by  the  passage  of  these  steam  bub- 
bles is  termed  boiling.  The  boiling-point  is  not  the 
same  in  different  liquids.  This  produces  the  variety 
we  see  in  the  forms  of  matter.  Some  vaporize  at 
ordinary  temperatures  ;  others  only  melt  at  the  very 
highest ;  while  the  gases  of  the  air  are  but  the  steam 
of  substances  which  vaporize  at  enormously  low 
temperatures.  The  boiling-point  of  water  depends 
on  three  circumstances. 

(1.)  The  purity  of  the  water. — Any  substance  which 
increases  the  cohesive  power  of  the  water  elevates 
the  boiling-point.  For  this  reason  salt  water  boils 


HEAT.  230 

at  a  higher  temperature  than  pure  water.  The  air 
dissolved  in  water  tends  by  its  elastic  force  to  sepa- 
rate the  molecules.  If  this  be  removed,  the  boiling- 
point  is  elevated  as  high  even  as  275°,  when  the 
water  is  converted  into  steam  with  explosive 
violence. 

(2.)  The  nature  of  the  vessel. — Water  will  boil  at  a 
lower  temperature  in  an  iron  than  in  a  glass  vessel. 
If  the  surface  of  the  glass  be  made  chemically  clean, 
the  boiling-point  is  elevated  still  higher.  This  seems 
to  depend  on  the  strength  of  the  adhesion  between 
the  water  and  the  vessel  in  which  it  is  contained. 

(3.)  The  pressure. — Any  pressure  upon  the  surface 
tends  to  keep  the  molecules  together,  and  so  raises 
the  boiling-point.  Water,  therefore,  boils  at  a  lower 
temperature  on  a  mountain  than  in  a  valley.  The 
temperature  of  boiling  water  at  Quito  is  90°,  and  on 
Mont  Blanc  84°  C.  The  variation  is  so  uniform, 
that  the  height  of  any  place  can  be  ascertained  with 
tolerable  accuracy  by  this  means.  A  difference  of 
1°  F.  is  produced  by  an  ascent  of  596  feet. 

The  influence  of  pressure  is  very  finely  illustrated 
by  the  following  experiment.  Boil  a  glass  flask  half 
full  of  water  for  some-  time.  Cork  it  quickly  and  then 
invert  it.  The  pressure  of  the  accumulated  steam 
will  soon  stop  all  ebullition.  A  few  drops  of  cold 
water  will  condense  the  steam,  and  boiling  will  com- 
mence again.  This  will  soon  be  checked,  but  can  be 
restored  as  before.  The  process  may  be  repeated 
until  the  water  cools  to  little  more  than  blood-heat. 


240 


NATURAL  PHILOSOPHY. 


Fig.  171. 


If  the  cork  be  air- 
tight, when  the  water 
is  quite  cold  it  will 
strike  with  a  sharp 
metallic  sound  as  it 
falls  from  one  end  of 
the  flask  to  the  other. 
The  cushion  of  air 
which  commonly 
breaks  the  fall  of  wa- 
ter is  here  removed. 
The  water-hammer  il- 
lustrates this  point 
yet  more  fully.  It 
consists  of  a  glass 
tube  half  full  of  wa- 
ter, from  which  the 
air  has  been  expelled  by  heat,  the  tube  being  sealed 
while  the  water  was  yet  boiling.  The  vacuum  is 
very  perfect ;  steam  may  be  produced  in  it  by  the 

Fig.  172. 


heat  of  the  hand,  and  the  water  falls  to  and  fro  with 
the  apparent  force  of  lead.  The  pulse-glass  shown 
in  the  figure  is  a  somewhat  similar  instrument. 


HEAT. 


241 


The  temperature  cannot  be  raised  above  the  boil- 
ing-point, unless  the  steam  is  confined,  however 
much  heat  may  be  applied.  The  extra  force  is 
entirely  occupied  in  expanding  the  water  into  steam. 
This  occupies  1,700  times  the  space,  and  is  of 
the  same  temperature  as  the  water  from  which 
it  is  made.  Over  900°  of  heat  become  latent  in 
this  process,  but  are  made  sensible  again  when 
the  steam  is  condensed.  The  common  method 
of  heating  by  steam  depends  upon  this  fact.  Steam 
is  invisible.  This  we  can  verify  for  ourselves  by 
examining  it  just  as  it  issues  from  the  spout  of  the 
tea-kettle.  It  soon  condenses,  however,  into  minute 
globules,  which,  floating  in  the  true  steam,  render 
the  vapor  apparently  visible. 

IY.  EVAPORATION  should  be  distinguished  from 
vaporization.  It  is  a  slow  formation  of  vapor,  which 
takes  place  at  all  ordinary  temperatures.  Ex. :  Wa- 
ter evaporates  slowly,  even  at  the  freezing-point. 
Clothes  dry  in  the  open  air  in  the  coldest  weather. 
The  wind  quickens  the  process,  because  it  drives 
away  the  moist  air  near  the  clothes  and  supplies  its 
place  with  dry  air.  Evaporation  is  also  hastened 
by  an  increase  of  surface  and  a  gentle  heat.  This 
principle  is  made  useful  in  the  arts  for  separating  a 
solid  from  the  liquid  which  holds  it  in  solution. 

Vacuum  pans  are  largely  employed  in  condensing 
milk,  in  the  manufacture  of  sugar,  etc.  They  are 
BO  arranged  that  the  air  above  the  liquid  in  the  ves- 
sel may  be  exhausted,  and  then  the  evaporation  takes 

11 


240 


NATURAL  PHILOSOPHY. 


Fig.  171. 


If  the  cork  be  air- 
tight, when  the  water 
is  quite  cold  it  will 
strike  with  a  sharp 
metallic  sound  as  it 
falls  from  one  end  of 
the  flask  to  the  other. 
The  cushion  of  air 
which  commonly 
breaks  the  fall  of  wa- 
ter is  here  removed. 
The  water-hammer  il- 
lustrates this  point 
yet  more  fully.  It 
consists  of  a  glass 
tube  half  full  of  wa- 
ter, from  which  the 
air  has  been  expelled  by  heat,  the  tube  being  sealed 
while  the  water  was  yet  boiling.  The  vacuum  is 
very  perfect ;  steam  may  be  produced  in  it  by  the 

Fig.  172. 


heat  of  the  hand,  and  the  water  falls  to  and  fro  with 
the  apparent  force  of  lead.  The  pulse-glass  shown 
in  the  figure  is  a  somewhat  similar  instrument. 


HEAT.  241 

The  temperature  cannot  be  raised  above  the  boil- 
ing-point, unless  the  steam  is  confined,  however 
much  heat  may  be  applied.  The  extra  force  is 
entirely  occupied  in  expanding  the  water  into  steam. 
This  occupies  1,700  times  the  space,  and  is  of 
the  same  temperature  as  the  water  from  which 
it  is  made.  Over  900°  of  heat  become  latent  in 
this  process,  but  are  made  sensible  again  when 
the  steam  is  condensed.  The  common  method 
of  heating  by  steam  depends  upon  this  fact.  Steam 
is  invisible.  This  we  can  verify  for  ourselves  by 
examining  it  just  as  it  issues  from  the  spout  of  the 
tea-kettle.  It  soon  condenses,  however,  into  minute 
globules,  which,  floating  in  the  true  steam,  render 
the  vapor  apparently  visible. 

IV.  EVAPORATION  should  be  distinguished  from 
vaporization.  It  is  a  slow  formation  of  vapor,  which 
takes  place  at  all  ordinary  temperatures.  Ex. :  Wa- 
ter evaporates  slowly,  even  at  the  freezing-point. 
Clothes  dry  in  the  open  air  in  the  coldest  weather. 
The  wind  quickens  the  process,  because  it  drives 
away  the  moist  air  near  the  clothes  and  supplies  its 
place  with  dry  air.  Evaporation  is  also  hastened 
by  an  increase  of  surface  and  a  gentle  heat.  This 
principle  is  made  useful  in  the  arts  for  separating  a 
solid  from  the  liquid  which  holds  it  in  solution. 

Vacuum  pans  are  largely  employed  in  condensing 
pailk,  in  the  manufacture  of  sugar,  etc.  They  are 
BO  arranged  that  the  air  above  the  liquid  in  the  ves- 
sel may  be  exhausted,  and  then  the  evaporation  takes 

11 


242 


NATURAL  PHILOSOPHY. 


place  very  rapidly,  and  at  so  low  a  temperature  that 
all  danger  of  burning  is  avoided. 

Fig.  173.  Evaporation      cools      a 

liquid  very  rapidly,  since 
sensible  heat  becomes  la- 
tent in  the  vapor.  Pious 
Mahometans  were  former- 
ly accustomed  to  place,  in 
niches  along  the  public 
streets,  porous  earthen- 
ware bottles  filled  with 
water,  to  refresh  the  thirsty 
travellers.  Water  may  even 
be  frozen  in  a  vacuum,  if  the  vapor  be  removed  as 
fast  as  formed.  Ice  is  manufactured  in  the  tropics 
by  machines  constructed  on  this  principle.  The 
greatest  artificial  cold  ever  known,  —220°  P.,  was 
produced  by  evaporating  in  a  vacuum  a  mixture  of 
liquid  nitrous  oxide  gas  and  bisulphide  of  carbon. 

SPHEROIDAL  STATE. — If  a  few  drops  of  water  be 
put  in  a  red-hot  metallic  cup,  they  will  gather  into 
a  globule,  which  will  dart  to  and  fro  over  the  surface 
with  little  diminution.  It  seems  to  rest  on  a  little 
cushion  of  steam,  which  supports  it  while  the  heated 
currents  of  air  drive  it  hither  and  thither.  If  the 
cup  be  allowed  to  cool,  after  a  little  the  water  will 
lose  its  spheroidal  form,  and  coming  into  direct 
contact  with  the  metal,  burst  into  steam  with  a 
slight  explosion.  This  principle  may  perhaps  ac- 
count for  some  unexplained  boiler  accidents.  By 


HEAT.  243 

moistening  our  finger,  we  can  touch  a  hot  flat-iron  with 
impunity.  The  water  assumes  the  above  state,  and 
thus  protects  the  flesh  from  injury.  Furnace-men 
often  dip  their  moistened  hands  into  molten  iron. 
Probably  the  accounts  handed  down  to  us  of  per- 
sons walking  unharmed  over  red-hot  ploughshares 
are  to  be  explained  in  this  manner. 

COMMUNICATION  OF  HEAT. 

Heat  tends  to  diffuse  itself  equally  among  all 
surrounding  bodies.  There  are  three  modes  of  dis- 
tribution. 

I.  CONDUCTION  is  the  process  of  heating  by  the  pas- 
sage of  heat  from  molecule  to  molecule.  Ex. :  Hold  one 
end  of  a  poker  in  the  fire,  and  the  other  end  soon  be- 
comes hot  enough  to  burn  the  hand.  Substances  vary 
in  their  power  of  conduction.  The  denser  bodies,  as 
the  metals,  possess  this  property  in  the  highest  de- 
gree. Of  the  ordinary  metals,  copper  is  the  best  con- 
ductor. Wood  is  a  poor  conductor,  especially  "  across 
the  grain."  Gases  are  the  poorest  conductors  ;  hence 
porous  bodies,  as  wool,  fur,  snow,  charcoal,  etc.,  which 
contain  within  them  large  quantities  of  air,  are  excel- 
lent non-conductors.  Kefrigerators  and  ice-houses 
have  double  walls,  filled  between  with  charcoal, 
sawdust,  or  other  non-conducting  substances.  Fire- 
safes  contain  plaster-of-paris.  Air  is  so  poor  a  con- 
ductor, that  persons  have  gone  into  ovens,  which 
were  so  hot  as  to  cook  meat  and  eggs  which  they 
carried  in  with  them  and  laid  on  the  metal  shelves  ; 


244 


NATURAL  PHILOSOPHY. 


yet,  so  long  as  they  did  not  themselves  touch  any 
good  conductor,  they  experienced  little  inconve- 
nience. Liquids  are  also  poor  conductors.  Ex.  : 
Hold  the  upper  end  of  a  test-tube  of  water  in  the 
flame  of  a  lamp.  The  water  nearest  the  blaze  will 
boil,  without  the  heat  being  felt  by  the  hand.  j 

All  adjacent  objects  have  the  same  temperature, 
yet  flannel  sheets  feel  warm,  and  linen  cold.  A 
marble  slab  seems  colder  than  the  woollen  carpet 
below  it.  If  we  touch  an  object  colder  than  we  are, 
it  abstracts  heat  from  us,  and  we  say  "  it  feels  cold ;" 
if  a  warmer  body,  it  imparts  heat  to  us,  and  we  say 
"  it  feels  warm."  These  various  effects  depend  en- 
tirely upon  the  relative  conducting  power  of  the 
different  substances.  Iron  feels  colder  than  feathers 
only  because  it  robs  us  faster  of  our  heat. 

II.  CONVECTION  is  the 
process  of  heating  by  circu- 
lation.— (1.)  Convection  of 
liquids.  Place  a  little 
sawdust  in  a  flask  of  wa- 
ter, and  apply  heat  at  the 
bottom.  We  shall  soon 
find  that  an  ascending 
and  a  descending  cur- 
rent are  established. 
The  water  near  the  lamp 
becoming  heated,  ex- 
pands and  rises.  The 
cold  water  above  sinks  to  take  its  place.  (2.) 


Fig.  174. 


HEAT.  245 

Convection  of  gases.  By  testing  with  a  lighted 
candle,  we  shall  find  that  at  the  bottom  of  a  door 
opening  into  cold  air,  there  is  a  current  setting  in- 
ward, and  at  the  top,  one  setting  outward.  The 
cold  air  in  a  room  flows  to  the  stove  along  the  floor, 
is  heated,  and  then  rises  to  the  ceiling.  All  methods 
of  heating  by  hot-air  furnaces  depend  upon  the 
principle  that  warm  air  rises. 

III.  RADIATION  is  the  process  of  heating  ~by  the 
transmission  of  rays  in  straight  lines.  Ah1  heat  from 
the  sun  comes  to  the  earth  in  this  manner.  A  hot 
stove  radiates  heat.  Rays  of  heat  do  not  elevate 
the  temperature  of  the  media  through  which  they 
pass.  When  the  motion  of  the  ether-waves  is 
stopped,  the  effect  is  felt.  Space  is  not  warmed 
by  the  sunbeam.  Meat  can  be  cooked  by  radia- 
tion, while  the  air  around  is  at  the  freezing-point. 
Eadiation  varies  in  different  bodies,  and  in  the  same 
body  under  different  circumstances.  A  rough  un- 
polished surface  is  a  better  radiator  than  a  smooth 
bright  one.  Extent  of  surface  increases  radiation. 
Air,  vapor,  and  glass  allow  luminous  rays  of  heat  to 
pass  through  them  readily.  Thus  the  heat  of  the 
sunbeam  easily  penetrates  our  atmosphere,  windows, 
etc.  But  the  earth,  and  various  objects  on  its  siu> 
face,  absorb  and  radiate  the  heat  back  again  as 
obscure  heat  in  long,  slow  waves.  These  have  no 
power  to  pass  the  watery  vapor  in  the  air  or  through 
glass.  They  are  thus  entangled,  and  kept  for  our 
use.  If  the  aqueous  vapor  were  removed  from  our 


2  4  6  NA  TURAL  PHIL  OSOPHY. 

air,  the  earth  would  become  uninhabitable,  through 
the  rapid  radiation.  On  the  desert  of  Sahara,  where 
"  the  soil  is  fire  and  the  wind  is  flame,"  the  dry  air 
allows  the  heat  to  escape  through  it  so  readily  that 
ice  is  sometimes  formed  at  night.  The  dryness  of 
the  air  at  great  elevations  accounts,  in  part,  for  the 
coldness  which  is  there  felt  so  keenly. 

Absorption  and  reflection  are  intimately  connected 
with  radiation.  A  good  absorber  is  also  a  good 
radiator,  but  a  good  reflector  can  be  neither.  Snow 
is  a  good  reflector  but  a  poor  absorber  or  radiator. 
Light  colors  absorb  less  and  reflect  more  than  dark 
colors.  White  is  the  best  reflector,  and  black  the 
best  absorber  and  radiator. 

THE  STEAM-ENGINE. 

When  steam  rises  from  water  at  a  temperature  of 
212°  it  has  an  elastic  force  of  15  Ibs.  per  square 
inch.  If  the  steam  is  confined  and  the  temperature 
raised,  the  elastic  force  is  rapidly  increased. 

THE  STEAM-ENGINE  is  a  machine  for  using  the  elas- 
tic force  of  steam  as  a  motive  power.  There  are 
two  classes  of  engines,  the  high-pressure  and  the  low- 
pressure.  In  the  former,  the  steam,  after  being  em- 
ployed to  do  its  work,  is  forced  out  into  the  air  ;  in 
the  latter,  it  is  condensed  in  a  separate  chamber  by 
a  spray  of  cold  water.  As  the  steam  is  condensed 
in  the  low-pressure  engine,  a  vacuum  is  formed  be- 
hind the  piston ;  while  the  piston  of  the  high-pres- 
sure engine  acts  against  the  pressure  of  the  air. 


HE A  T. 


247 


Fig.  175, 


The  elastic  force  of  the  steam  must  be  15  Ibs.  per 
square  inch  greater  in  the  latter  case.  In  the  figure 
we  have  repre- 
sented the  pis- 
ton and  connect- 
ing pipes  of  an 
engine.  The 
steam  from  the 
boiler  passes 
through  the  pipe 
into  the  steam- 
chest,  as  indi- 
cated by  the 
arrow.  The  slid- 
ing-valve  worked  by  the  rod  li  lets  the  steam  into  the 
cylinder  alternately  above  and  below  the  piston, 
which  is  thus  made  to  play  up  and  down  by  the  ex- 
pansive force. 

The  Governor  is  an  apparatus  for  regulating  the 
supply  of  steam.  A  B  is  the  axis 
around  which  the  heavy  balls  E 
and  D  revolve.  When  the  ma- 
chine is  going  too  fast  the  balls 
fly  out  by  centrifugal  force  and 
shut  off  a  portion  of  the  steam ; 
when  too  slowly,  they  fall  back, 
and,  opening  the  valve,  let  on  the 
steam  again. 

The  high-pressure  engine  in  the   form  commonly 
used  is  shown  in  the  frontispiece.     A  represents  the 


Fig.  176. 


248 


NATVRAL  PHILOSOPHY, 


cylinder,  B  the  steam-chest,  C  the  throttle- valve  in 
the  pipe  through  which  steam  is  admitted  from  the 
boiler,  D  the  governor,  E  the  band-wheel  by  which 
the  governor  is  driven,  F  the  pump,  G  the  crai*k,  I 
the  connector  which  is  attached  to  a  the  cross-head, 
H  the  eccentric  rod  (h  in  Fig.  175)  which  works  the 
sliding-valve  in  the  steam-chest,  K  the  governor- 
valve,  S  the  shaft  by  which  the  power  is  conveyed 
to  the  machinery.  The  cross-head,  a,  slides  to 
and  fro  in  a  groove,  and  is  fastened  to  the  rod  which 
works  the  piston  in  the  cylinder  A.  The  expansive 
force  of  the  steam  is  thus  communicated  to  a,  thence 
to  I,  by  which  the  crank  is  turned.  The  heavy  fly- 
wheel, by  its  inertia,  serves  to  render  the  movement 
of  the  machinery  uniform. 

METEOKOLOGY. 

The  air  always  contains  moisture.  The  amount 
it  can  receive  depends  on  the  temperature  ;  warm 
air  absorbing  more,  and  cold  air  less.  At  75°  the 
vapor  is  sometimes  so  dense  that  in  a  cubic  yard  of 
atmosphere  there  is  a  cubic  inch  of  water.  At  50° 
half  that  quantity  must  be  deposited.  When  the 
air  at  any  temperature  contains  all  the  vapor  it  can 
hold,  it  is  said  to  be  saturated;  any  fall  of  tempera- 
ture will  then  cause  a  part  of  the  vapor  to  be  condensed. 
Most  of  the  phenomena  of  rain,  hail,  dew,  etc.,  de- 
pend on  this  principle. 

A  change  in  density  produces  a  change  in  temperature. 
Place  a  little  tinder  at  the  end  of  the  piston  of  the 


HEAT. 


249 


Fig.  177. 


fire-syringe  shown  in  the  figure.     By  forcing  down 
the    handle    and    compressing    the    air,   sufficient 
heat    is    liberated  to 
ignite  the  tinder.     On 
the    other    hand,    in 
experiments  with   the 
Air-pump    we    notice 
that  as  the  air  is  rare- 
fied, a  mist  gathers  in 
the  glass  receiver.  This 
shows  that  the  atmos- 
phere is  cooled  by  its 
expansion,  and  so  de- 
posits its  vapor.     The 
warm    air    from     the 
earth   ascending    into 
the  upper  regions,  is 
rarefied  and  cooled  in 
the  same  manner.     Its 
vapor  is  condensed  into 
clouds,  and  often  falls 
as  rain.     Owing  to  this 
expansion  of  the  air,  there  is  a  gradual  diminution 
of  the  temperature  as  the  altitude  is  increased,  at 
the  rate  of  about  1°  for  every  300  feet.     Even  in 
tropical  climates  the  tops  of  high  mountains   are 
covered  with  perpetual  snow.     At  the  equator  the 
snow-line  is  15,000  feet  above  the  level  of  the  sea. 
Should,  however,  a  blast  of  cold  air  descend  from  a 
lofty  height,  it  would  become  so  condensed  in  fall- 

11* 


250  NA  TVRAL  PHIL OSOPHY. 

ing,  and  its  temperature  thereby  so  elevated,  that  it 
would  produce  no  injurious  effect  on  vegetation. 

DEW. — The  grass  at  night,  becoming  cooled  by 
radiation,  condenses  upon  its  surface  the  vapor  of 
the  air.*  Dew  will  gather  most  freely  upon  those 
objects  that  are  the  best  radiators,  as  they  will  the 
soonest  become  cool.  Thus  grass,  leaves,  etc.,  which 
need  the  most,  get  the  most.  It  will  not  form  on 
windy  nights,  because  the  air  is  constantly  chang- 
ing and  does  not  become  cool  enough  to  deposit  its 
moisture.  In  tropical  regions  the  nocturnal  radia- 
tion is  often  so  great  as  to  admit  of  the  formation  of 
ice.  In  Bengal  this  is  accomplished  by  exposing 
water  in  shallow  earthen  dishes  resting  on  rice-straw. 
The  most  dew  collects  on  a  clear,  cloudless  night. 
In  many  countries,  by  its  abundance,  it  supplies  the 
place  of  rain,  as  in  Chili,  Arabia,  etc.  When  the 
temperature  of  plants  falls  below  32°,  the  vapor  is 
frozen  upon  them  directly,  and  is  called  hoar-frost. 

FOGS  are  formed  when  the  temperature  of  the  air 
falls  below  the  dew-point  (i.  e.,  the  temperature  at 
which  dew  is  deposited).  They  are  found  mainly 


*  Dew  was  anciently  thought  to  possess  many  wonderful  prbp- 
erties.  Baths  in  this  precious  liquid  were  said  to  conduce  greatly 
to  beauty.  It  was  collected  for  this  purpose,  and  for  the  use  of 
the  alchemists  in  their  weird  experiments,  by  spreading  fleeces  of 
wool  upon  the  ground.  Laurens,  a  philosopher  of  the  middle 
ages,  claimed  that  dew  is  ethereal,  so  that  if  we  should  fill  a  lark's 
egg  with  it  and  lay  it  out  in  the  sun,  immediately  on  the  rising 
of  that  luminary,  the  egg  will  fly  off  into  the  air !  This  expert* 
ment  is  best  performed  with  a  goose's  egg. 


HEAT. 


251 


on  low  grounds  and  in  the  vicinity  of  rivers,  ponds, 
etc.,  where  the  abundance  of  moisture  keeps  the  air 
constantly  saturated. 

CLOUDS  differ  from  fogs  only  in  their  elevation  in 
the  atmosphere.  They  are  formed  when  a  "  warm, 
humid  wind  penetrates  a  cold  air,  or  a  cold  wind  a 
warm,  humid  air."  Mountains  are  "  cloud-capped" 

Fig.  178. 


Different  kinds  of  clouds — Ibird  indicates  the  nimbus,  2  birds  the  stratus, 
3  birds  the  cumulus,  and  4  birds  the  cirrus  cloud. 

because  the  warm  air  rising  from  the  valley  is  con- 
densed upon  their  cold  summits.  Clouds  are  con- 
stantly falling  by  their  weight,  but  as  they  melt 
away  in  the  warm  air  below,  by  condensation  they 
increase  above. 


252 


NATURAL  PHILOSOPHY. 


The  nimbus  cloud  is  a  dark-colored  cloud  from 
which  rain  is  falling. 

The  stratus  cloud  is  composed  of  broad,  widely- 
extended  cloud-belts,  sometimes  spread  over  the 
whole  sky.  It  is  the  lowest  cloud,  and  often  rests 
on  the  earth.  It  is  the  night-cloud. 

The  cumulus  cloud  is  made  up  of  large  cloud- 
masses  looking  like  snow-capped  mountains  piled  up 
along  the  horizon.  It  forms  the  summits  of  pillars 
of  vapor,  which,  streaming  up  from  the  earth,  are 
condensed  in  the  upper  air.  It  is  the  day-cloud. 
When  of  small  size  and  seen  only  near  mid-day,  it  is 
a  sign  of  fair  weather. 

The  cirrus  (curl)  cloud  consists  of  light,  fleecy 
clouds  floating  high  in  air.  It  is  believed  to  be 
formed  of  spiculse  of  ice  or  flakes  of  snow. 

The  cirro-cumulus  is  formed  by  small,  distinct, 
rounded  portions  of  the  cirrus  cloud,  which  separate 
from  each  other,  leaving  a  clear  sky  between. 
Sailors  call  this  a  "  mackerel  sky."  It  accompanies, 
warm,  dry  weather. 

The  cirro-stratus  is  produced  when  the  cirrus 
cloud  spreads  out  into  long,  slender  strata.  It  fore- 
bodes storms. 

The  cumulo-stratus  presents  the  peculiar  forms 
called  "  thunder-heads."  It  is  caused  by  a  blend- 
ing of  the  cumulus  with  the  stratus,  and  is  a  precur- 
sor of  thunder-storms. 

BAIN  is  vapor  condensed  by  the  sudden  cooling  of 
the  air  in  the  upper  regions.  At  a  low  temperature 


HEAT.  253 

the  vapor  is  frozen  directly  into  snow.  This* may 
melt  before  it  reaches  the  earth,  and  fall  as  rain.  A 
sudden  draught  of  cold  air  into  a  heated  ball-room 
has  sometimes  produced  a  miniature  snow-storm. 
The  wonderful  variety  and  beauty  of  snow-crystals 
are  illustrated  in  the  accompanying  figure. 

Fig.  179. 


WINDS  are  produced  by  variations  in  the  tempera- 
ture of  the  air.  The  atmosphere  at  some  point  is 
expanded,  rises,  and  colder  air  flows  in  to  supply  its 
place.  This  produces  currents.  The  land  and  sea 
breezes  of  tropical  islands  are  caused  by  the  unequal 
specific  heat  of  land  and  water.  During  the  day  the 
land  becomes  more  highly  heated  than  the  water, 
and  hence  toward  evening  a  sea-breeze  sets  in  from 
the  ocean.  At  night  the  land  cools  faster  than  the 


254  NATURAL  PHILOSOPHY. 

water,  and  so  toward  morning  a  land-breeze  sets  out 
from  the  land.  Trade-winds  are  so  named  because 
by  their  regularity  they  favor  commerce.  A  vessel 
on  the  Atlantic  Ocean  will  sometimes,  without  shifting 
a  sail,  set  steadily  before  this  wind  from  Cape  de 
Yerde  to  the  American  coast.  The  air  about  the 
equator  is  highly  heated,  and,  rising  to  the  upper 
regions,  flows  off  north  and  south.  The  cold  air 
near  the  poles  sets  toward  the  equator  to  fill  its 
place.  If  the  earth  were  at  rest  this  would  make  an 
upper  warm  current  flowing  from  the  equator,  and 
a  lower  cold  current  flowing  toward  it.  As  the 
earth  is  revolving  on  its  axis  from  west  to  east,  the 
under  current  starting  from  the  poles  is  constantly 
coming  to  a  part  moving  faster  than  itself.  It 
therefore  lags  behind.  When  it  reaches  the  north 
equatorial  regions  it  lags  so  much  that  it  becomes  a 
current  from  the  northeast,  and  in  the  south  equa- 
torial regions  a  current  from  the  southeast. 

OCEANIC  CURRENTS  are  produced  in  a  similar  man- 
ner. The  water  which  is  heated  by  the  vertical  sun 
of  the  tropics  rises  and  flows  toward  the  poles.  The 
Gulf  Stream,  which  issues  from  the  Gulf  of  Mexico, 
carries  the  heat  of  the  Caribbean  Sea  across  the 
Northern  Atlantic  to  the  shores  of  ^Scotland  and 
Norway.  This  tropical  river  flowing  steadily  through 
the  cold  water  of  the  ocean,  rescues  England  from 
the  snows  of  Labrador.  Should  it  by  any  chance 
break  through  the  Isthmus  of  Panama,  Great  Britain 
would  be  condemned  to  eternal  glaciers. 


HEAT.  255 

VAKIOUS  FORMS  AND  ADAPTATIONS  OF  WATER. — The 
great  specific  heat  of  water  adapts  it  to  exercise  a 
marked  influence  on  climate.  Warm  winds  sweep- 
ing northward  meet  the  colder  air  of  the  temperate 
regions  and  deposit  their  moisture.  The  latent  heat 
which  the  vapor  absorbed  in  the  sunny  South  is  set 
free,  to  temper  the  severity  of  our  snowy  climate. 
Thus,  aerial  and  oceanic  currents  constitute  a  water 
circulation  which  is  a  natural  steam  apparatus  on 
the  grandest  scale,  since  it  has  a  boiler  at  the  equa- 
tor, and  steam-pipes  running  over  the  entire  globe. 
Water  also  equalizes  the  climate.  It  tends  to  pre- 
vent sudden  changes  of  weather.  In  the  summer  it 
absorbs  vast  quantities  of  heat,  which  it  gives  off  in 
the  fall  to  moderate  the  approach  of  winter.  In  the 
spring  the  melting  ice  and  snow  drink  in  the  warmth 
of  the  sunbeam,  which  else  might  prematurely  coax 
forth  the  tender  buds.  Since  so  much  heat  is  re- 
quired to  melt  the  ice  and  snow,  they  dissolve  very 
slowly,  and  thus  prevent  in  a  measure  the  disastrous 
floods  which  would  inevitably  follow,  if  they  passed 
quickly  into  the  liquid  state. 

Water  contains  air,  which  is  necessary  for  the 
support  of  fish.  Just  here  occurs  one  of  those 
happy  coincidences  which  frequently  startle  the 
reverent  searcher  in  Nature.  Were  water  deprived 
of  this  air,  it  would  be  liable  to  explode  at  any 
moment  when  it  happened  to  be  heated  much  above 
212°.  Every  stove-boiler  would  need  a  thermom- 
eter. A  teakettle  would  then  require  as  careful 


256 


NATURAL  PHILOSOPHY. 


watching  as  now  to  attend  a  steam-engine,  and 
our  kitchens  would  witness  frequent  and  most  dis- 
astrous explosions.  As  it  is,  when  the  tempera- 
ture rises  above  212°,  the  extra  heat  passes  off 
quietly  and  safely.  Water  expands  with  heat,  like 
other  liquids,  and  contracts,  on  cooling,  down  to 
39°  F.  Then  it  slowly  expands  until  it  reaches  32° 
E.,  when  it  freezes.  The  bursting  of  water-pipes 
and  pails  is  a  familiar  example  of  this  exception. 
Under  the  operation  of  the  general  law,  the  water 
at  the  surface  radiating  its  heat  and  becoming 
chilled,  would  contract  and  fall  to  the  bottom,  while 
the  warm  water  below  would  rise  to  the  top.  This 
process  would  continue  until  the  freezing-point  was 
reached,  when  the  whole  mass  would  instantly  so- 
lidify into  ice.  Our  lakes  and  rivers  would  thus 
freeze  solid  every  winter.  This  would  be  fatal  to 
fish  and  aquatic  vegetation.  In  the  spring,  the  ice 
would  not,  as  now,  buoyant  and  light,  float  and  melt 
in  the  direct  sunbeam,  but,  lying  at  the  bottom, 
would  be  protected  by  the  non-conducting  water 
above.  The  longest  summer  would  not  be  sufficient 
to  thaw  the  deeper  bodies  of  water.  Here  we  see 
another  instance  of  prudent  foresight.  An  exception 
is  made  to  prevent  these  disastrous  consequences.* 
The  cold  water  expands  and  rises  to  the  top,  thus 
protecting  the  warm  water  beneath,  while  ice  itself, 

*  Certain  metals— iron,  bismuth,  etc. — are  also  an  exception  to 
the  general  law.  This  fact  adapts  them  for  castings.  Is  not  this 
equally  a  thoughtful  provision  for  our  wants  ? 


HEAT.  257 

being  a  non-conductor,  preserves  the  temperature 
of  the  water  quite  uniform  during  the  entire  winter. 

Water,  in  freezing,  has  a  tendency  to  free  itself 
from  impurities.  This  furnishes  a  means  of  obtain- 
ing fresh  water  in  Arctic  regions.  McClintock  found 
that  on  each  successive  freezing  the  ice  was  purer, 
until,  on  the  fourth  time,  he  obtained  drinking-water. 
If  a  barrel  of  vinegar  freezes,  we  shall  find  the  acid 
collected  in  a  little  mass  at  the  centre  of  the  ice. 

When  the  dew  collects  at  night  sufficiently  to 
form  a  covering  upon  the  plants,  being  a  non-con- 
ductor, it  stops  further  radiation  of  heat.  Thus,  by 
a  nice  provision,  the  effect  of  radiation  checks  the 
radiation  itself,  as  soon  as  the  wants  of  the  thirsty 
vegetation  are  supplied. 

Water  distills  from  the  ocean  and  land  as  vapor,  at 
one  time  cooling  and  refreshing  the  air,  at  another 
moderating  its  wintry  rigor.  It  condenses  into 
clouds,  which  shield  the  earth  from  the  direct  rays 
of  the  sun,  and  protect  against  excessive  radiation. 
It  falls  as  rain,  cleansing  the  air  and  quickening 
vegetation  with  renewed  life.  It  descends  as  snow, 
and,  like  a  coverlet,  wraps  the  grass  and  tender  buds 
in  its  protecting  embrace.  It  bubbles  up  in  springs, 
invigorating  us  with  cooling,  healing  draughts  in  the 
sickly  heat  of  summer.  It  purifies  our  system,  dis- 
solves our  food,  and  keeps  our  joints  supple.  It 
flows  to  the  ocean,  fertilizing  the  soil,  and  floating 
the  products  of  industry  and  toil  to  the  markets  of 
the  world.  (See  Chemistry,  pp.  56-63.) 


258  NATURAL  PHILOSOPHY. 

Practical  Questions.— \.  Why  will  one's  hand,  on  a  frosty  morning, 
freeze  to  a  metallic  door-knob  sooner  than  to  one  of  porcelain  ?  2.  Why  doea 
a  piece  of  bread  toasting  curl  up  on  the  side  exposed  to 'the  fire  ?  3.  Why  do 
double  windows  protect  from  the  cold  ?  4.  Why  do  furnace-men  wear  flan- 
nel shirts  in  summer  to  keep  cool,  and  in  winter  to  keep  warm  ?  5.  Why  do 
we  blow  our  hands  to  make  them  warm,  and  our  soup  to  make  it  cool  ?  6. 
Why  does  snow  protect  the  grass?  7.  Why  does  water  "boil  away"  more 
rapidly  on  some  days  than  on  others  ?  8.  What  causes  the  crackling  sound 
in  a  stove  when  a  fire  is  lighted  ?  9.  Why  is  the  tone  of  a  piano  higher  in  a 
cold  room  than  in  a  warm  one  ?  10.  Ought  an  inkstand  to  have  a  large  or  a 
small  mouth  ?  11.  Why  is  there  a  space  left  between  the  ends  of  the  rails  on 
a  railroad  track  ?  12.  Why  is  a  person  liable  to  take  cold  when  his  clothes 
are  damp?  13.  What  is  the  theory  of  corn-popping?  14.  Could  vacuum- 
pans  be  employed  in  cooking?  15.  Why  does  the  air  feel  so  chilly  in  the 
spring,  when  snow  and  ice  are  melting  ?  16.  Why  in  freezing  ice-cream  do 
we  put  the  ice  in  a  wooden  vessel  and  the  cream  in  a  tin  one?  17.  Why 
does  the  temperature  generally  moderate  when  snow  falls  ?  18.  What  causes 
the  singing  of  a  teakettle.  Ans.  The  escaping  steam  is  thrown  into  vibra- 
tion by  the  peculiar  shape  of  the  spout.  19.  Why  does  sprinkling  a  floor 
with  water  cool  the  air  ?  20.  How  low  a  degree  of  temperature  can  be  marked 
by  a  mercurial  thermometer?  21.  If  the  temperature  be  70°  F.,  what  is  it 
C.?  22.  Will  dew  form  on  an  iron  bridge  ?  On  a  plank  walk  ?  23.  Why 
will  not  corn  pop  when  very  dry  ?  24.  The  interior  of  the  earth  being  a 
melted  mass,  why  do  we  get  the  coldest  water  from  a  deep  well  ?  25.  Ought 
the  bottom  of  a  teakettle  to  be  polished  ?  26.  Which  boils  the  sooner,  milk 
or  water  ?  27.  Is  it  economy  to  keep  our  stoves  highly  polished  ?  28.  If  a 
thermometer  be  held  in  a  running  stream,  will  it  indicate  the  same  tempera- 
ture that  it  would  in  a  pailful  of  the  same  water  ?  29.  Which  makes  the  bet- 
ter "  holder,"  woollen  or  cotton  ?  3O.  Which  will  give  out  the  more  heat,  a 
plain  stove  or  one  with  ornamental  designs  ?  31.  Does  dew  fall  ?  32.  What 
causes  the  "  sweating"  of  a  pitcher?  33.  Why  is  evaporation  hastened  in  a 
vacuum?  34.  Does  stirring  the  ground  around  plants  aid  in  the  deposition 
of  dew  ?  35.  Why  does  the  snow  at  the  foot  of  a  tree  melt  sooner  than  that 
in  the  open  field  ?  36.  Why  is  the  opening  in  a  chimney  made  to  decrease 
in  size,  from  bottom  to  top?  37.  Will  tea  keep  hot  longer  in  a  bright  or  a 
dull  teapot  ?  38.  What  causes  the  snapping  of  wood  when  laid  on  the  fire  ? 
Ans.  The  expansion  of  the  air  in  the  cells  of  the  wood.  39.  Why  is  one's 
breath  visible  on  a  cold  day?  40.  What  gives  the  blue  color  to  air?  Ans. 
The  vapor  it  contains  reflects  the  blue  light  of  the  sunbeam  ?  41.  Why  is 
light-colored  clothing  cooler  in  summer  and  warmer  in  winter  than  dark  ? 
42.  How  does  the  heat  at  two  feet  from  the  fire  compare  with  that  at  a  dis- 
tance of  four  feet  ?  43.  Why  does  the  frost  remain  later  in  the  morning  upon 
some  objects  than  upon  others  ?  44.  Is  it  economy  to  use  green  wood  ?  45. 
Why  does  not  green  wood  snap  ?  46.  Why  will  a  piece  of  metal  dropped  into 
a  glass  or  porcelain  dish  of  boiling  water  increase  the  ebullition  ?  47.  Which 
can  be  ignited  the  more  quickly  with  a  burning-glass,  black  or  white  paper  ? 
48.  Why  does  the  air  feel  colder  on  a  windy  day  ?  49.  In  what  did  the  mir- 
acle of  Gideon's  fleece  consist?  50.  Could  a  burning  lens  be  made  of  ice  ? 
51.  Why  is  an  iceberg  frequently  enveloped  by  a  fog?  52.  Would  dew 
gather  more  freely  on  a  rusty  stove  than  on  a  bright  kettle?  53.  Why  is  a 
clear  night  colder  than  a  cloudy  one  ?  54 .  Why  is  no  dew  formed  on  cloudy 


*  rhat  power  which,  like  a  potent  spMt,  guidM 
The  sea- wide  wanderers  over  distant  tides, 
Inspiring  confidence  where'er  they  roam, 
By  indicating  still  the  pathway  home ;  — 
Through  nature,  quickened  by  the  solar  beam, 
Invests  each  atom  with  a  force  supreme,  » 

Directs  the  cavern'd  crystal  in  its  birth, 
And  frames  the  mightiest  mountains  of  thfr  earth; 
Each  leaf  and  flower  by  its  strong  law  restrains, 
And  binds  the  monarch  Man  within  its  mystic  chains." 

Htnin 


MA  GNETIC  ELECTRICITY.  3  6 1 

THALES,  one  of  the  seven  wise  men,  knew  that 
when  amber  is  rubbed  with  silk  it  will  attract  light 
bodies,  as  straw,  leaves,  etc.  This  property  was 
considered  so  marvellous  that  amber  was  supposed 
to  possess  a  soul.  From  the  Greek  name  of  this 
substance  (elektron)  our  word  electricity  is  derived. 
The  electrical  force  manifests  itself  in  five  different 
forms — (1)  Magnetic  electricity ;  (2)  Fractional  or  stati- 
cal electricity ;  (3)  Galvanic,  voltaic,  or  dynamic  elec- 
tricity ;  (4)  Thermal  electricity ;  (5)  Animal  electricity. 
These  are  intimately  connected ;  their  laws  are 
strikingly  related ;  they  produce  many  effects  in 
common  ;  and  each  can  give  rise  to  the  other. 


MAGNETIC  ELECTRICITY. 

Magnetism  treats  of  the  properties  of  magnets. 

A  MAGNET  is  a  body  which  has  the  power  of  at- 
tracting iron.  The  term  is  derived  from  the  fact 
that  an  ore  of  iron  possessing  this  property  was  first 
found  at  Magnesia,  in  Asia  Minor.  Natural  magnets 
are  generally  known  as  lodestones  (Saxon,  laedan, 
to  lead).  The  one  worn  by  Sir  Isaac  Newton 
weighed  only  3  grains,  yet  it  was  able  to  lift  746 
grains,  or  nearly  250  times  its  weight.  Their  power 
does  not  increase  in  proportion  to  their  size.  One 
brought  from  Moscow  to  London  weighed  125  Ibs., 
but  could  support  only  about  200  Ibs.  The  artificial 
magnet  consists  of  a  magnetized  steel  bar ;  if  straight, 


262  NA  TURAL  PHIL  OSOPHY. 

it  is  called  a  bar  magnet ;  if  bent  into  the  shape 
of  the  letter  U,  a  horse-shoe  magnet.  A  piece 
of  soft  iron,  called  the  armature,  is  placed  on  the 
end. 

The  Poles. — If  we  insert  a  magnet  in  iron-filings, 
they  will  cling  chiefly  to  its  extremities,  which  are 

Fig.  180. 


termed  the  poles.  The  magnetic  force  will  be  ex- 
erted even  through  an  intervening  body.  Lay  a 
sheet  of  paper  on  a  magnet  and  sprinkle  iron-filings 
upon  it.  They  will  collect  in  curious  lines,  the  mag- 


MAGNETIC  ELECTRICITY. 

Fig.  181. 


netic  curves,  radiating  from  the  poles.  If  a  small  bar 
magnet  be  suspended  so  as  to  swing  freely,  one  pole 
will  point  toward  the  north  and  the  other  toward 


264  NATURAL  PHILOSOPHY. 

the  south.  The  north  pole  of  the  magnet  is  called 
the  positive  (  +  ),  and  the  south  pole,  the  nega- 
tive (— ).  A  magnet  thus  poised  constitutes  a  mag- 
netic needle.  If  we  hold  a  magnet  near  a  magnetic 
needle,  we  shall  find  that  the  south  pole  of  one  at- 
tracts the  north  pole,  and  repels  the  south  pole  of 
the  other.  This  proves  the  law  of  magnetic  attrac- 
tion and  repulsion — "  Like  poles  repel,  and  unlike 
poles  attract." 

MAGNETIC  INDUCTION  is  the  power  a  magnet  pos- 
sesses to  develop  magnetism  in  iron.  If  a  piece  of 
soft  iron  be  brought  near  a  magnet,  it  immediately 
assumes  the  magnetic  state,  which,  however,  it  loses 
on  being  removed.  In  steel  the  change  is  perma- 
nent. The  end  of  the  bar  next  to  the  south  pole  of 
the  magnet  becomes  the  north  pole  of  the  new  mag- 
net, and  vice  versa.  When  opposite  states  are  thus 
developed  in  the  opposite  ends  of  a  body,  it  is  said 
Fi  183  to  be  polarized. 

Whenever  any  ob- 
ject is  attracted 
by  a  magnet,  it  is 
supposed  first  to 
be  made  a  mag- 
net (polarized)  by 
induction,  and 
then  the  attraction 
consists  merely  in  that  of  unlike  poles  for  each 
other.  Thus  we  may  suspend  from  a  magnet  a 
chain  of  rings  held  together  by  magnetic  attraction. 


MA  GNETIC  ELECTRICITY.  2  6  5 

Each  link  is  a  magnet  with  its  north  and  south 
poles.  Each  particle  of  the  tuft  of  filings  in  Fig.  180 
is  a  distinct,  perfect  magnet.  A  magnet  does  not 
lose  any  force  by  inducing  magnetism.  It  rather 
gains  strength  by  the  reflex  influence  of  the  new 
magnet.  An  armature  acts  in  this  manner  to 
strengthen  a  magnet.  If  we  break  a  magnet  even 
into  the  smallest  fragments,  each  part  will  have  a 
north  and  a  south  pole.  It  is  explained  by  sup- 
posing that  every  molecule  of  iron  contains  two 
kinds  of  electric  force  which  neutralize  each  other. 
"When  magnetized  they  are  separated,  but  do  not 
leave  the  molecule  in  which  they  reside.  Each 
molecule  is  thus  polarized,  the  two  halves  assuming 
opposite  magnetic  states,  as  shown  in  the  figure. 
The  light  half  of  each  Pig  184 

little  circle  represents 
the  positive,  and  the 
dark  the  negative  side.  All  the  molecules  exert 
their  negative  force  in  one  direction,  and  their  posi- 
tive in  the  other.  The  forces  thus  neutralize  each 
other  at  the  centre,  but  manifest  themselves  at  the 
ends  of  the  magnet. 

How  TO  MAKE  A  MAGNET. — The  following  method 
is  an  excellent  one.     Place  the  in-  Fig  185< 

ducing  magnet  on  the  unmagnet- 
ized  one,  as  shown  in  the  figure, 
and  draw  it  from  one  end  to  the 
other  several  times,  always  carry- 
ing it  back  through  the  air  in  a 
circle  to  the  starting-point. 


266 


NATURAL  PHILOSOPHY. 


THE  COMPASS  is  a  magnetic  needle  used  by  mari- 
ners, hunters,  surveyors,  etc.  It  is  very  delicately 
poised  over  a  card  on  which  the  "  points  of  the 
compass"  are  marked.  The  needle  does  not  often 
point  directly  N.  and  S.  The  "  line  of  no  variation" 
as  it  is  called,  runs  in  an  irregular  course  through 

Fig.  186. 


the  United  States  from  Cape  Lookout  across  Lake 
Erie  to  Hudson's  Bay.  East  of  this,  the  variation 
(declination)  is  toward  the  west,  and  west  it  is  toward 
the  east.  The  needle  is  subject  also  to  daily  and 
yearly  variations,  as  well  as  those  which  require 
centuries  to  complete.  The  needle  is,  however, 
"  true  to  the  pole,"  although  it  shifts  thus  every 
hour  in  the  day.  It  does  so  only  in  obedience  to 
the  laws  which  control  its  action. 


MAGNETIC  ELECTRICITY. 


267 


Fig.  187. 


Fig.  188. 


WHY  THE  NEEDLE  POINTS  NORTH  AND  SOUTH. — The 
earth  is  a  great  magnet.  This  gives  direction  to 
the  needle.  Variations  which  are  constantly  taking 
place  in  the  terrestrial  magnetism  produce  corres- 
ponding changes  in  the  needle.  Suppose  a  magnet 
N  S  passing  through  the  centre  of  a  small  globe. 
The  needle  s  n  will 
hang  parallel  to  it, 
as  in  Fig.  187,  its 
north  pole  being 
attracted  by  the 
south  pole  of  the 
magnet,  and  vice 
versa.  If  the  globe 
be  turned,  Fig.  188, 
the  north  pole  of 

the  needle  will  bend — dip,  as  it  is  termed — down- 
ward. If  the  globe  be  turned  in  the  other  direction, 
the  south  pole  of  the  needle  will  dip  in  the  same 
manner.  Similar  phenomena  are  noticed  in  the 
compass.  At  the  equator  it  is  horizontal,  but 
dips  whenever  taken  north  or  south.  An  unmag- 
netized  needle,  if  carefully  poised,  in  our  latitude, 
on  being  magnetized,  immediately  settles  down,  as 
if  the  north  end  were  the  heavier.  This  difficulty 
is  remedied  by  making  the  north  end  of  the  needle 
lighter,  and  also  by  suspending  a  little  weight  upon 
the  south  end.  The  reverse  is  true  in  the  southern 
hemisphere. 

A  dipping-needle  is  poised  as  shown  in  Fig.  189. 


268 


NATURAL  PHILOSOPHY, 


Fig.  189. 


At  the  equator  it  hangs  horizontally,  but  declines  as 
it  is  carried  north,  until,  at  a 
place  near  Hudson's  Bay,  as 
discovered  by  Captain  Boss  in 
1832,  it  becomes  vertical.  This 
point  is  called  the  North  mag- 
netic pole.  Strangely  enough, 
it  does  not  coincide  with  the 
geographical  pole.  The  South 
magnetic  pole  has  not  yet  been 
found.  From  the  experiments 
we  have  made,  we  see  that 
the  end  of  the  needle  which 
points  toward  the  N.  pole  of 
the  earth,  is  really  its  S.  pole. 
THE  EAKTH  INDUCES  MAGNETISM. — All  iron  bars 
standing  vertically  (which  in  this  latitude  is  not  far 
from  the  line  of  the  dip)  possess  slight  magnetic 
properties.  The  upright  parts  of  an  iron  fence, 
lightning  rods,  standards  of  chairs  and  desks,  etc., 
on  being  tested  by  the  magnetic  needle,  will  be 
found  to  possess  north  polarity  in  the  end  next  the 
ground,  and  south  polarity  in  the  other.  The  polar- 
ity of  the  lodestone  has  doubtless  been  caused  in 
this  manner  in  the  lapse  of  ages. 


FBICTIONAL  ELECTRICITY. 


269 


FBICTIONAL  ELECTRICITY.* 

This  is  electricity  developed  by  friction.  One's 
hair  often  crackles  under  a  gutta-percha  comb.  A 
cat's  back,  when  rubbed  in  a  dark  room,  emits 
sparks.  In  cold,  frosty  weather,  a  person,  by  shuf- 
fling about  in  his  stocking- feet  upon  the  carpet,  can 
develop  so  much  electricity  in  his  body  that  he  can 
ignite  a  jet  of  gas  by  simply  applying  his  finger 
to  it. 

THE  ELECTROSCOPE  is  an  instrument  for  detecting 
the  presence  of  electricity.  Bend  a  glass  tube,  and 
suspend  from  it  a  couple  of  Fig.  190. 

pith-balls,  as  shown  in  the 
figure.  Two  strips  of  gold- 
leaf,  hung  in  a  glass  jar 
(Fig.  191),  form  a  more  del- 
icate test.  This  instrument 
is  so  sensitive,  that  a  slight 
flap  of  a  silk  handkerchief 
against  the  cover  will  cause  the  leaves  to  diverge. 

Two  KINDS  OF  ELECTRICITY. — If  a  warm,  dry  glass 
tube  (a  lamp-chimney  will  answer)  be  rubbed  with 
a  silk  handkerchief,  a  crackling  sound  is  heard.  If 
the  tube  be  held  near  the  face,  we  shall  experience  a 
sensation  like  that  given  by  a  cobweb.  The  tube 

*  The  term  static  is  applied  to  frictional  electricity,  and  dy- 
namic to  galvanic.  The  former  indicates  a  force  at  rest;  the 
latter,  one  in  motion. 


270 


NATURAL  PHILOSOPHY. 


will  attract  bits  of  paper,  straw,  feathers,  etc.     Pre- 
sent it  to  the  pith-balls  in  the  electroscope  (Fig. 
Fig.  191.  190).    They  will 

be  attracted  for 
an  instant,  and 
will  then  fly  from 
the  tube  and 
from  each  other, 
apparently  in 
the  utmost  dis- 
gust. Electrify 
a  stick  of  seal- 
ing-wax and  pre- 
sent it  to  the 
balls.  They  will 
act  in  the  same 
manner.  If  we 
touch  -one  ball 
to  the  excited 
glass,  and  the  other  to  the  excited  wax,  they  will 
not,  as  before,  fly  from  each  other,  but  will  rush  to- 
gether at  once.  Present  the  glass  to  a  ball :  it  will 
fly  off  when  electrified.  Present  the  glass  again,  and 
it  will  be  repelled.  Substitute  the  wax,  and  it  will 
be  attracted.  Offer  now  the  glass,  and  it  will  eagerly 
bound  toward  what  it  just  .before  spurned.  If  the 
glass  be  held  on  one  side  of  a  ball  and  the  wax  on 
the  other,  it  will  fly  between  the  two,  carrying  the 
electricity  back  and  forth.  From  this  we  conclude  (1), 
that  there  are  two  kinds  of  frictional  as  of  magnetic 


PRICT10NAL  ELECTRICITY. 


271 


electricity ;  and  (2),  like  electricities  repel  each  other, 
and  unlike  attract.  The  electricity  from  the  glass  is 
termed  vitreous  or  positive  [  +  ],  and  that  from  the 
wax,  resinous  or  negative  [  — ]. 

In  the  following  list,  each  substance  becomes 
positively  electrified  when  rubbed  with  the  body 
following  it ;  but  negatively,  with  the  one  preceding 
it.  (Ganot.) 

1.  Cat1  s  fur.  5.  Cotton.  9.  Shellac.  13.  Caoutchouc. 

2.  Flannel.  6.  Silk.  10.  Resin.  14.  Gutta-percha. 

3.  Ivory.  7.  The  hand.  11.  The  metals.  15.  Gun-cotton. 

4.  Glass.  8.  Wood.  12.  Sulphur. 

THEORY  OF  ELECTRICITY. — Of  the  nature  of  elec- 
tricity we  know  little.  The  positive  and  negative 
forces  exist  in  every  body  in  a  state  of  equilibrium. 
When  this  is  disturbed  by  friction,  chemical  action, 
etc.,  both  are  set  free.  We  cannot  develop  one 
without  the  other.  The  opposite  kinds  manifest 
themselves  at  opposite  parts  of  the  surface,  as  in  a 
magnet ;  it  is  therefore  called  a  polar  force.  The 
slightest  causes  disturb  the  electric  equilibrium. 
"  In  cutting  a  slice  of  meat,  there  may  pass  between 
the  steel  knife  and  silver  fork  enough  electricity  to 
move  the  needle  of  a  telegraph."  Yet  the  delicate 
balance  of  the  opposing  forces  is  so  soon  readjusted 
that  we  are unconscious  of  the  change. 

CONDUCTORS  AND  INSULATORS. — A  body  which  al- 
lows the  electric  force  to  pass  freely  through  it  is 
termed  a  conductor ;  one  which  does  not,  is  called  a 
non-conductor,  or  insulator.  Copper  is  one  of  the 


272  NA  TURAL  PHIL  OSOPHY. 

best  conductors,  and  hence  it  is  used  in  all  electrical 
experiments.  Glass  is  one  of  the  best  insulators. 
A  body  is  said  to  be  insulated  when  it  is  supported 
by  some  non-conducting  substance,  usually  glass. 
The  air,  when  dry,  is  a  non-conductor,  but  when 
moist  becomes  a  good  conductor.  Hence,  the  elec- 
tricity can  be  retained  on  an  insulated  body  in  a 
dry  atmosphere,  but  is  soon  dissipated  in  a  damp 
one.  Electricity  can  be  collected  only  by  means  of 
insulation.  It  can  be  developed  by  rubbing  an  iron 
rod,  but  is  lost  as  fast  as  formed,  by  passing  off 
through  the  metal  to  the  hand.  A  glass  rod  does 
not  conduct  it  to  the  body,  so  it  is  retained  until  it 
gradually  dissipates  in  the  air.  The  following  list 
contains  the  most  common  conductors  and  insu- 
lators. 

Best  Conductors. 

Metals.  Vegetables.  Ice.  Glass. 

Charcoal.  Animals.  Dry  Wood.  Wax. 

Flame.  Linen.  Caoutchouc.  Sulphur. 

Minerals.  Cotton.  Dry  Paper.  Amber. 

Water.  Dry  Wood.  Air.  Shellac. 

Iron.  Ice.  Silk.  Best  Insulators. 

THE  ELECTKICAL  MACHINE  consists  (1)  of  a  glass 
wlied  turned  by  a  crank  ;  (2)  of  a  pair  of  rubbers  cov- 
ered with  leather  and  spread  with  an  amalgam  (a 
mixture  of  tin,  zinc,  and  mercury)  which  hastens  the 
development  of  electricity  ;  (3)  of  a  comb  or  fork 
with  fine  points,  since  pointed  bodies  always  favor 
the  reception  or  dispersion  of  electricity  ;  (4)  of  a 
prime  conductor — a  brass  cylinder  insulated  by  a 
glass  standard  so  that  the  electricity  cannot  pass  to 


FBICTIONAL  ELECTBICITY. 


273 


the  ground,  and  rounded  at  the  ends  so  that  it  may 
not  escape  too  rapidly  into  the  atmosphere. 

At  the  commencement,  the  whole  apparatus  is  in  a 
state  of  equilibrium.  By  the  friction,  positive  elec- 
tricity is  developed  on  the  glass,  and  negative  on  the 


Fig. 


rubber.  The  negative  escapes  along  the  chain  to  the 
ground — the  common  reservoir.  The  positive,  kept  on 
the  glass  by  the  silk  flaps,  is  carried  around  to  the 
points.  Here  it  attracts  the  negative  electricity  of  the 
prime  conductor,  and  the  two  forces,  clashing  together, 
form  tiny  sparks.  The  positive  electricity  naturally 
present  in  the  prime  conductor  is  thus  left  insulated, 
and  tKe  prime  conductor  is  said  to  be  charged  with  posi- 
tive electricity.  If  the  negative  conductor  be  insulated, 


274 


NATURAL  PHILOSOPHY. 


Fig.  193. 


the  rubber  will  soon  become  charged  with  negative 
electricity,  and  the  action  of  the  machine  will  nearly 
cease.  If  the  air  be  dry,  the  rubber  freshly  spread 
with  amalgam,  and  the  glass  well  rubbed  with  warm 
flannel,  a  sharp  crackling  noise  will  be  heard,  flashes 
will  follow  the  wheel  around,  while  sparks  can  be 
obtained  from  the  prime  conductor  at  a  distance  of 
several  inches.  The  pith-ball  electroscope,  when 
charged  and  repelled  by  the  prime  conductor,  will  be 
quickly  attracted  by  the  rubber.  This  indicates  the 
opposite  electricities  in  them. 

INDUCTION. — The  influence  of  an  electrified  body 
over  other  bodies  near  it  is  termed  electrical  induc- 
tion. Thus,  let  a 
small  insulated 
conductor  be 
placed  near  the 
ball  at  the  end  of 
the  prime  con- 
ductor of  an  elec- 
trical machine. 
On  charging  the 
prime  conductor 
the  motion  of  the 
pith-balls  will 
show  that  the  small  conductor  has  also  become 
charged.  On  testing  with  the  electroscope,  we  shall 
find  that  the  end  next  the  positive  prime  conductor 
is  negative,  and  the  other  end  positive.  As  oppo- 
site electricities  are  thus  developed  at  the  opposite 


FRICTIONAL  ELECTRICITY. 


275 


extremities  of  the  conductor,  it  is  polarized.     Place 
several  conductors,  as  shown  in  Fig.  194,  connecting 


Fig.  1&4. 


the  copper  ball  at  the  right  with  the  positive  pole, 
and  the  one  at  the  left  with  the  negative  pole  of  the 
electrical  machine.  The  conductors  will  be  charged 
and  polarized  by  induction. 

Faraday's  theory  of  induction  assumes  (1)  that  the 
electricity  acts  between  the  different  molecules  of  a 
body,  as  between  the  different  conductors  in  the  last 
experiment — that  each  molecule  becomes  polarized, 
and  in  turn  polarizes  its  neighbors,  and  that  thus  at 
last  every  molecule  has  opposite  electricities  on  its 
opposite  sides ;  (2)  that  the  molecules  of  non-conduct- 
ors become  polarized  and  retain  their  electricities, 
while  the  molecules  of  conductors  become  polarized 
and  discharge  their  electricities  into  the  adjacent 
molecules.  The  positive  force  thus  passing  from 
one  molecule  to  another  of  a  conductor  accumulates 
at  one  end,  and  the  negative,  moving  in  the  oppo- 
site direction,  collects  at  the  other  end.  Let  P  (Fig. 
195)  represent  the  end  of  the  positive  conductor  and 
N  that  of  the  small  conductor  in  Fig.  193  ;  and 


2.76 


NATURAL  PHILOSOPHY. 


© 


Fig.  196. 


let  the  small  circles  represent  molecules  of  air  lying 

between  the  two — the  lighter  half  indicating  the 

FI°-  195    positive  and  the  darker  half  the  negative  side. 

OThe  molecules  of  air  being  non-conducting, 
on  being  polarized  from  the  influence  of  P, 
®  ®  ®   the  prime  conductor,  retain  their  electrici- 
®  ties,  but  polarize  each  other  in  succession 

until  N  is  reached.  This  being  a  conduct- 
ing body,  its  molecules  impart  their  elec- 
tricity from,  one  to  the  other,  until  the  nega- 
tive electricity  collects  at  one  end  and  the 
positive  at  the  other. 

ATTKACTION  AND  REPULSION. — Every  case  of  attrac- 
tion or  repulsion  is  preceded  by  induction.     "  The 

electric  chime"  illustrates 
this  very  prettily.  It  con- 
sists of  three  bells,  two  of 
which,  c  and  b,  are  hung 
by  brass  chains,  while  the 
middle  one  is  insulated 
above  by  a  silk  cord,  and 
connected  below  with  the 
earth  by  a  chain.  The 
balls  between  them  are 
also  insulated.  The  outer  bells  becoming  charged 
with  positive  electricity  from  the  prime  conductor, 
polarize  the  balls  by  induction  through  the  inter- 
vening air.  The  balls  being  then  attracted  to  the 
bells,  are  charged  and  immediately  repelled.  Swing- 
ing away,  they  strike  against  the  middle  bell,  dis- 


FRICTIONAL  ELECTRICITY. 


277 


Fig.  197. 


charge  their  electrical  force,  and  are  forthwith  at- 
tracted again.  Flying  to  and  fro,  they  ring  out  a 
merry,  electrical  song.  The  dancing  image  is  another 
illustration.  It  consists  of  a 
little  pith-ball  figure  placed 
between  two  metallic  plates, 
the  upper  one  hanging  from 
the  prime  conductor,  and  the 
lower  one  connected  with  the 
earth.  The  dance  is  conduct- 
ed in  a  remarkably  lively 
manner  by  alternate  attrac- 
tion and  repulsion.* 

THE  LEYDEN  JAB  consists  of  a  glass  jar  coated 
inside  and  outside,  nearly  to  the  top,  with  tinfoil. 
It  is  fitted  with  a  cover  of  Fig.  ins. 

baked  wood,  through  which 
passes  a  wire  with  a  knob 
at  the  top,  and  below,  a 
chain  extending  to  the  inner 
coating.  The  jar  is  charged 
by  bringing  the  knob  near 
the  prime  conductor  of  the 
electrical  machine,  while  the 
outer  coating  communicates 
freely  with  the  earth.  Bright  sparks  will  then  leap 
in  rapid  succession  to  the  inner  coating,  while  simi- 

*  A  very  slow  motion  should  be  given  to  the  electrical  wheel, 
and  a  pin  thrust  into  the  heel  of  the  image  will  add  much  to  the 
stamp  of  the  tiny  feet. 


278 


NATURAL  PHIL OSOPHY. 


lar  ones  will  pass  off  from  the  outer  coating.  The 
jar  is  discharged  by  holding  one  knob  of  the  "  dis- 
charger" E,  upon  the  outer  coating,  and  the  other 
upon  the  knob  of  the  jar.  The  equilibrium  will  be  re- 
stored with  a  sharp  snap  and  a  brilliant  flash.  Mi- 
nute particles  are  detached  from  the  solid  conduct- 
ors, and,  burning,  give  color  and  brilliancy  to  the 
spark.* 

Explanation. — The  charging  of  a  jar  with  electrici- 
ty is  entirely  different  from  the  process  of  filling 
one  with  water.  The  glass  can  as  well  be  in  the 
form  of  a  pane.  The  only  essentials  are  ttvo  con- 
ducting surfaces  separated  by  a  non-conducting  body. 
The  tinfoil  acts  only  as  a  conductor  to  convey  the 
electricity.  This  is  finely  illustrated  by  the  "  Ley  den 
jar  with  movable  coatings,"  which  may  be  charged 
and  then  taken  apart.  Very  little  electricity  can  be 
obtained  from  the  glass,  either  of  the  tin  coatings, 
or  any  two  of  the  parts  combined.  When  put  to- 
gether again,  the  jar  can  be  discharged  in  the  usual 


*  Professor  Muschenbroek,  of  Leyden,  discovered  the  princi- 
ple of  the  Leyden  jar  in  the  following  curious  way.  While  experi- 
menting, he  held  a  bottle  of  water  to  the  prime  conductor  of  his 
electrical-machine.  Noticing  nothing  peculiar,  he  attempted  to 
investigate  its  condition.  Holding  the  bottle  with  one  hand,  he 
happened  to  touch  the  water  with  the  other,  when  he  received  a 
shock  so-  unexpected,  and  so  unlike  anything  he  had  ever  felt 
before,  that  he  was  filled  with  astonishment  It  was  two  days 
before  he  recovered  from  his  fright.  A  few  days  afterward,  in  a 
letter  to  a  friend,  the  Professor  innocently  remarked,  that  he 
would  not  take  another  shock  for  the  whole  kingdom  of  France. 


FRIUTIONAL  ELECTRICITY.  270 

manner.  Eig.  199  represents  an  enlarged  section 
of  the  side  of  a  Ley  den  jar :  1  indicates  the  inner 
coating ;  2,  the  outer  coating,  and  Fig.  m 

the  circles  between,  the  molecules      /  2, 

of  glass.  The  sparks  of  positive 
electricity  from  the  prime  con- 
ductor are  distributed  by  the  inner 
coating,  over  the  jar.  The  mole- 
cules of  glass  are  polarized,  while 


oe>e> 
occ 


the  outer  coating  becomes  charged 
by  induction  with  negative  electricity.  A  quantity 
of  positive  electricity  corresponding  to  the  positive 
received  by  the  inner  coating  escapes  from  the  outer 
coating.  If  the  jar  be  insulated  so  that  this  is  unable 
to  leave,  the  passage  of  the  sparks  will  soon  cease.  If 
a  finger  be  held  near  the  outer  coating,  a  spark  will 
leap  to  it  every  time  one  enters  the  jar.  The  jar, 
therefore,  when  charged,  contains  no  more  electricity 
than  in  its  natural  state.  It  is  only  differently  dis- 
tributed. 

THE  ELECTRICITY  is  ON  THE  SURFACE. — Each  mole- 
cule within  the  surface  of  a  solid,  insulated  conductor 
gives  up  its  electricity  with  equal  freedom  in  every 
direction ;  therefore  it  cannot  become  charged. 
Each  molecule  on  the  surface,  however,  receiving 
electricity  from  the  particles  behind  it,  and  having 
non-conducting  particles  of  air  before  it,  must  be- 
come charged.  A  bomb-shell  can  therefore  hold  as 
much  electricity  as  a  cannon-ball. 

THE  EFFECT  OF  POINTS. — A  pointed  wire  held  near 


28o  NATURAL  PHILOSOPHY. 

the  prime  conductor  will  quietly  draw  off  all  its 
electricity,  which  will  be  seen  apparently  clinging 
to  the  point  like  a  little  glowing  star.  If  we  fasten 
a  pointed  wire  to  the  prime  conductor,  it  will  dis- 
charge the  electricity  in  a  brush  of  flame,  silently, 
but  so  rapidly  that  even  the  pith-balls  will  not  reveal 
its  presence  in  the  conductor.  If  we  hold  one  cheek 
near  the  point,  we  shall  feel  a  current  of  air  setting 
away  from  it.  This  is  strong  enough  to  deflect  the 
flame  of  a  candle.  The  particles  of  air  near  the  point 
become  polarized,  are  attracted,  give  up  their  negative 
electricity,  and,  being  charged  with  positive  electricity, 
are  repelled ;  new  ones  take  their  place,  and  thus  a 
current  is  established.  The  electric  whirl,  mounted  on 
the  prime  conductor  (Fig.  192),  illustrates  this  action. 
As  each  molecule  of  air  is  repelled  from  a  point,  it 
reacts  with  equal  force  against  the  point.  This  is  suf- 
ficient to  set  the  light  wire-wheel  in  rapid  rotation. 

ATMOSPHERIC  ELECTRICITY. — If,  with  the  friction 
upon  a  small  glass  wheel,  so  much  electricity  is 
developed,  what  immense  quantities  must  be  pro- 
duced by  moving  masses  of  air,  clouds,  etc. !  Added 
to  this,  are  the  effects  of  heat,  chemical  changes,  and 
the  varied  processes  of  nature — all  of  which  disturb 
the  electrical  equilibrium.  The  air,  therefore,  is  al- 
most constantly  electrified.  In  clear  weather  it  is 
in  a  positive  state,  but  in  foul  weather  it  changes 
rapidly  from  positive  to  negative,  and  vice  versa. 
Dr.  Livingstone  tells  us  that  in  South  Africa  the  hot 
wind  which  blows  over  the  desert  is  so  highly  elec- 


FRICTIONAL  ELECTRICITY.  2Sl 

trified,  that  a  bunch  of  ostrich  feathers  held  for  a 
few  seconds  against  it  becomes  as  strongly  charged 
as  if  attached  to  an  electrical-machine,  and  will 
clasp  the  hand  with  a  sharp,  crackling  sound. 

LIGHTNING  is  only  the  discharge  of  a  Ley  den  jar 
on  the  grand  scale  upon  which  Nature  performs  her 
operations.*  Two  clouds  charged  with  opposite 
electricities,  and  separated  by  the  non-conducting 
air,  approach  each  other.  "When  the  tension  be- 
comes sufficient  to  overcome  the  resistance,  the  two 
forces  rush  together  with  a  blinding  flash  and  ter- 
rific peal.  The  lightning  moves  along  the  line 
where  there  is  the  least  resistance,  and  so  describes 
a  zigzag  course.  If  we  can  trace  the  entire  length, 
we  call  it  chain-lightning ;  if  we  only  see  the  flash 
through  intervening  clouds,  it  is  sheet-lightning  •  and 
if  it  is  the  reflection  of  distant  discharges,  we  term 
it  heat-lightning.  The  report  is  caused  by  the  clash- 
ing of  the  atoms  of  displaced  air.  The  rolling  of 
the  thunder  is  produced  by  the  reflection  of  the 

*  The  identity  of  lightning  and  frictional  electricity  was  dis- 
covered by  Franklin.  He  made  a  kite  of  a  silk  handkerchief,  and 
fixed  at  the  top  a  pointed  wire.  He  elevated  this  during  a  thun- 
der-storm, tying  at  the  end  of  the  hemp  string  a  key,  and  then 
insulating  the  whole  by  fastening  it  to  a  post  with  a  long  piece 
of  silk  lace.  On  presenting  his  knuckles  to  the  key,  he  obtained 
a  spark.  So  great  was  his  joy,  that  he  is  said  to  have  burst  into 
tears.  He  afterward  charged  a  Leyden  jar,  and  performed  other 
electrical  experiments  in  this  way.  These  attempts  were  attended 
with  very  great  danger.  A  few  years  after,  Prof.  Richman  drew 
in  this  manner  from  the  clouds  a  ball  of  blue  fire  as  large  as  a 
man's  fist.  It  struck  him  lifeless. 


282  NATURAL  PHILOSOPHY. 

sound  from  distant  clouds.  Sometimes  the  clouds 
and  the  earth  become  charged  with  opposite  elec- 
tricities, separated  by  the  non-conducting  air.  The 
spark  from  the  discharge  of  this  huge  Leyden  jar  is 
a  bolt  that  often  causes  fearful  destruction. 

Fig.  200. 


The  Aurora  Borealis — "  Northern  lights" — is  prob- 
ably caused  by  the  passage  of  electricity  through 
the  rarefied  atmosphere  of  the  upper  regions.  It 
may  be  beautifully  imitated,  on  a  small  scale,  by 
passing  a  succession  of  sparks  from  the  prime  con- 
ductor through  a  long  glass  tube  from  which  the  air 
is  nearly  exhausted.  The  intimate  relation  between 
the  aurora  and  magnetism  is  shown  from  "the  fact 
that  the  magnetic  needle  is  disturbed  when  the  au= 


FRIOT10NAL  ELECTRICITY.  283 

rora  is  visible,  and  seems  to  tremble  as  the  stream- 
ers dart  to  and  fro.  The  telegraph  is  frequently 
worked  by  the  current  of  electricity  which  passes 
along  the  wire  on  these  occasions,  thus  for  a  time 
dispensing  with  the  line-batteries.  Geisskr's  Tubes 
are  filled  with  rarefied  gases,  and  then  sealed.  When 
a  current  of  electricity  is  passed  through  them,  the 
richest  tints  and  variegated  bands  of  color  are  exhib- 
ited. Gassiot's  Cascade  consists  of  a  glass  goblet 
coated  with  tinfoil  on  the  inside.  This  is  placed  on  the 
air-pump,  and  covered  with  a  receiver  which  has  a 
sliding-rod  passing  through  the  top.  The  air  is  then 
exhausted,  and  the  rod  brought  into  contact  with 
the  tinfoil.  One  conductor  of  the  electrical-machine 
is  connected  with  the  rod,  and  the  other  with  the 
pump-plate.  The  electricity  will  flow  over  the  sides 
of  the  cup  in  a  shower  of  soft  undulations  and  deli- 
cate blue  light. 

Lightning-rods  were  invented  by  Franklin.  They 
are  based  on  the  principle  that  electricity  always 
seeks  the  best  conductor.  The  rod  should  be  point- 
ed at  the  top  with  some  metal  which  will  not  easily 
corrode.  If  constructed  in  separate  parts,  they  should 
be  securely  jointed.  The  lower  end  should  extend 
into  water,  or  else  deep  into  the  damp  ground,  be- 
yond a  possibility  of  any  drought  rendering  the 
earth  about  it  a  non-conductor,  and  be  packed  about 
with  ashes  or  charcoal.  If  the  rod  is  of  iron,  it 
needs  to  be  much  larger  than  if  of  copper,  which  is 
a  better  conductor.  Every  elevated  portion  of  the 


284  NA  TURAL  PHIL OSOPUY. 

building  should  be  protected  by  a  separate  rod 
Chimneys  in  which  fire  is  constantly  kept  need  espe- 
cial care,  because  of  the  ascending  column  of  vapor 
and  smoke.  Water  conductors,  tin  roofs,  etc.,  should 
be  connected  with  the  damp  ground  or  the  light- 
ning-rod, that  they  may  aid  in  conveying  off  the 
electricity.  The  value  of  a  lightning-rod  consists, 
most  of  all,  in  its  power  of  quietly  restoring  the 
equilibrium  between  the  earth  and  the  clouds.  By 
erecting  lightning-rods,  we  thus  lessen  the  liabilities 
of  a  sudden  discharge.  Providence  has  provided 
largely  against  this  catastrophe.  "  God  has  made 
a  harmless  conductor  in  every  leaf,  spire  of  grass, 
and  twig.  A  common  blade  of  grass,  pointed  by 
Nature's  exquisite  workmanship,  is  three  times  more 
effectual  than  the  finest  cambric  needle,  and  a  single 
pointed  twig  than  the  metallic  point  of  the  best- 
constructed  rod."  Every  drop  of  rain,  and  every 
snow-flake,  falls  charged  with  the  electric  force,  and 
thus  quietly  disarms  the  clouds  of  their  terror.  The 
balls  of  electric  light,  called  by  sailors  " St.  Elmos 
fire"  which  sometimes  cling  to  the  masts  and 
shrouds  of  vessels,  and  the  flames  seen  to  play 
about  the  points  of  bayonets,  indicate  the  quiet 
escape  of  the  electric  force  from  the  earth  toward 
the  clouds. 

VELOCITY  OF  ELECTRICITY. — The  duration  of  the 
flash  has  been  estimated  at  one-millionth  of  a  sec- 
ond. Some  idea  of  its  instantaneousness  can  be 
formed  from  the  fact  that  the  spokes  of  a  wheel  re- 


FRICTION AL  ELECTRICITY.  2  g  g 

volved  so  rapidly  as  to  become  invisible  by  daylight 
can  be  distinctly  seen  by  the  spark  from  a  Leyden 
jar.  The  trees  swept  by  the  tempest,  when  seen  by 
a  flash  of  lightning,  seem  motionless,  while  a  can- 
non-ball, in  swift  flight,  appears  poised  in  mid-air. 
Wheatstone  considered  the  velocity  of  lightning 
through  a  copper  wire  to  be  288,000  miles  per 
second. 

EFFECTS  OF  ERICTIONAL  ELECTRICITY. — I.  Physical. 
— Discharges  from  a  large  battery  will  melt  rods  of 
the  various  metals,  perforate  glass,  split  wood,  mag- 
netize steel  bars,  etc.  1.  Let  a  person  stand  upon 
an  insulated  stool  and  become  charged  from  the 
prime  conductor.  His  hair,  through  repulsion,  will 
stand  erect  in  a  most  ludicrous  manner.  On 
presenting  his  hand  to  a  little  ether  contained  in 

Fig.  201. 
/^ 


a  spoon,  a  spark  leaping  from  his  extended  finger 
will  ignite  it.  If  he  hold  in  his  hand  an  icicle, 
the  spark  will  readily  dart  from  it  to  the  liquid. 
2.  A  card  held  between  the  knob  of  a  Leyden  jar 
and  that  of  the  discharger,  will  be  punctured  by  the 
spark..  3.  A  piece  of  steel  may  be  magnetized  by 
the  discharge  from  an  ordinary  Leyden  jar.  Wind 
a  covered  copper  wire  around  a  steel  bar,  as  in  Fig. 
201,  or  simply  enclose  a  needle  in  a  small  glass 


286  NATURAL  PUILOSO'PUY. 

tube   around  which  the  wire   may  be  wound.     On 
passing  the  spark  through  the  wire,  the  needle  will 
attract  iron-filings.     4.  When  strips  of  tinfoil  are 
Fig.  202.  pasted  on  glass,  and  figures  of  va- 

rious curious  patterns  cut  from 
them,  the  electric  spark  leaping  from 
one  to  the  other  presents  a  beauti- 
ful appearance.  The  diamond-  Ley- 
den  jar  and  the  spiral  tube  illustrate 
these  effects  in  a  brilliant  manner. 
5.  If  a  battery  be  discharged 
through  a  wire  too  small  to  conduct  the  spark,  the 
electricity  is  changed  to  heat,  and  if  sufficiently 
small,  the  wire  will  be  fused  into  globules  or  dissi- 
pated in  smoke. 

The  fact  that  the  electric  force  is  thus  converted 
into  vibrations  of  heat  and  light,  would  seem  to  in- 
dicate that,  like  them,  it  is  only  a  mode  of  motion. 

II.  Chemical  effects. — The  "  electric  gun"  is  filled 
with  a  mixture  of  oxygen  and  hydrogen  gases.     A 
spark  causes  them  to  combine  with  a  loud  explosion 
and  form  water.   .  The  sulphurous  smell  which  ac- 
companies  the   working   of   an   electrical-machine, 
and  is  noticed  in  places  struck  by  lightning,  is  owing 
to  the  production  vof  ozone,  a  peculiar  form  of  the 
oxygen  of  the  air.     (See  Chemistry,  p.  38.) 

III.  Physiological   effects. — A   very   slight   charge 
from  a  Ley  den  jar  produces  a  contraction  of  the 
muscles  and  a  spasmodic  sensation  in  the  wrist.     A 
stronger  one  affects  the  body,  and  becomes  painful 


GALVANIC  ELECTRICITY. 


and  even  dangerous.  The  shock  may  be  given  to  a 
large  number  of  persons  simultaneously  by  joining 
hands.  The  Abbe  Nollet  once  shocked  in  this  way 
a  regiment  of  1,500  soldiers. 


GALVANIC  ELECTRICITY. 

Galvanic  or  Yoltaic  electricity  is  produced  by 
chemical  action.*  These  names  are  given  in  honor 
of  the  two  Italian  philosophers  who  made  the  first 
discoveries  in  this  branch  of  electricity. 

GALVANI' s  DISCOVEEY. — In  the  year  1790  Galvani 
was  engaged  in  some  experiments  on  animal  elec- 
tricity. For  this  purpose  he  used  frogs'  legs  as 
electroscopes.  He  had  hung  several  of  these  upon 
copper  hooks  from  the  iron  railing  of  the  balcony, 
in  order  to  see  what  effect  the  atmospheric  elec- 
tricity might  have  upon  them.  He  noticed,  to  his 
surprise,  that  when  the  wind  blew  them  against  the 
iron  supports,  the  legs  were  convulsed  as  if  in  pain. 
After  repeated  experiments,  Galvani  concluded  that 
this  effect  was  produced  by  what  he  termed  animal 
electricity,  that  this  electricity  was  different  from 
that  caused  by  friction,  and  that  he  had  discovered 
the  agent  by  which  the  will  controls  the  muscles. 

*  The  pupil,  on  recalling  the  definition  of  Natural  Philosophy, 
will  readily  perceive  that  galvanic  electricity  is  a  connecting  link 
between  philosophy  and  chemistry.  Its  cause  is  chemical,  while 
its  effects  are  both  chemical  and  philosophical.  It  is  oftentimes 
ranked  as  a  part  of  what  is  termed  Chemical  Physics. 


288 


NATURAL  PHILOSOPHY. 


YOLTA'S  DISCOVERY. — Volta  rejected  the  idea  of 
animal  electricity,  and  after  27  years  of  incessant 
study,  discovered  that  the  frog  was  not  the  source  of 
the  electricity,  but  "  only  a  moist  conductor,  and 
FIR.  203.  was  not  as  good  as  a  wet  rag  for  that  pur- 
pose." He  applied  this  view  to  the  con- 
struction of  "  Yolta's  pile."  This  is  com- 
posed of  plates  of  zinc  and  copper,  between 
which  are  laid  pieces  of  flannel  moistened 
with  an  acid  or  saline  solution.  We  can 
easily  form  a  simple  voltaic  pile  by  placing 
a  silver  coin  between  our  teeth  and  upper  lip,  and  a 
piece  of  zinc  under  our  tongue.  On  pressing  the 
edges  of  the  two  metals  together,  we  shall  perceive  a 
peculiar  taste,  while  a  flash  of  light  will  pass  before 
the  closed  eyes.  Yolta  Relieved  that  the  contact 
of  two  dissimilar  metals  develops  electricity.  His 
theory  has  given  place  to  the  chemical  one  which  we 
shall  now  notice. 

THE  SIMPLE  GALVANIC  CIRCUIT. — If  we  place  a 
strip  of  zinc  in  a  cup  of  water  well  acidulated  with 
sulphuric  acid  (oil  of  vitriol),  a 
chemical  action  will  at  once 
commence.  Little  bubbles  of 
hydrogen  gas  will  gather  on  the 
metal,  while  the  zinc  rapidly 
dissolves.  If  we  now  immerse 
the  zinc  in  mercury,  the  surface 
will  become  as  bright  as  a  mir- 
ror. Replace  the  strip  in  the 


GALVANIC  ELECTRICITY.  289 

cup,  and  the  acid  will  have  no  effect  upon  it.  The 
reason  of  this  action  is  not  understood,  but  all  zinc 
used  in  galvanic  batteries  is  thoroughly  and  frequent- 
ly amalgamated  in  this  manner.  Now  put  a  strip  of 
copper  in  the  acid.  As  long  as  the  two  metals  remain 
separate  no  change  takes  place,  but  as  soon  as  they 
touch,  or  are  connected  by  wires  as  in  the  figure, 
chemical  action  begins,  and  the  bubbles  of  hydro- 
gen gather  upon  the  copper  instead  of  the  zinc  as  be- 
fore. The  copper  will  not  be  changed,  but  the  zinc 
will  waste  away.  As  soon  as  the  wires  are  sepa- 
rated the  action  ceases,  and,  in  the  dark,  a  minute 
spark  is  seen. 

The  ends  of  the  wires  are  termed  poles  or  elec- 
trodes. The  copper  pole  is  positive  and  the  zinc 
negative.  (These  names  may  be  easily  remembered 
if  we  associate  the  p's  with  copper  and  positive,  and 
the  n's  with  zinc  and  negative.)  Platinum  strips  are 
often  fastened  to  the  ends  of  the  wires  to  act  as 
electrodes,  in  order  to  withstand  the  corrosive  liquids 
in  which  we  may  wish  to  place  the  poles.  The  join- 
ing of  the  wires  is  termed  closing  the  circuit,  and 
separating  them  breaking  the  circuit.  Two  metallic 
plates  combined  in  this  manner  form  a  voltaic  pair. 
The  two  metals  must  be  dissimilar  (one  positive  and 
the  other  negative),  and  must  be  immersed  in  a 
liquid  which  is  capable  of  producing  a  chemical  ef- 
fect on  only  one  of  them.  If  both  are  equally  acted 
upon,  no  current  will  be  established,  since  the  elec- 
tricity set  free  by  each  will  neutralize  that  developed 

13 


200  NA  TURAL  PHILOSOPH  Y. 

by  the  other.  The  metal  which  sets  free  the  elec- 
tricity is  termed  the  positive,  and  the  other  the 
negative  plate.* 

The  chemical  change  which  takes  place  in  the 
voltaic  pair  may  be  very  simply  explained  as  follows  : 
jEach  molecule  of  water  is  composed  of  an  atom  of 
oxygen  and  two  of  hydrogen  ;  the  former  unites  with 
the  zinc,  forming  oxide  of  zinc.  The  sulphuric  acid 
combines  with  this,  making  sulphate  of  zinc,  which 
dissolves  in  the  water.  The  hydrogen  being  set  free 
rises  to  the  surface  and  escapes.  (For  the  replace- 
ment theory,  see  Chemistry,  p.  51.) 

Why  the  hydrogen  comes  off  from  the  copper  plate.  — 
For  simplicity  of  illustration,  we  shall  suppose  a  row 
Fig  205          °^  wa^er  molecules  t  extending  from 
the  zinc  to  the  copper  plate.     The 
negative  oxygen  of  the  molecule  of 
water  nearest  the  positive  zinc  is  at- 
tracted to  that  plate,  while  the  posi- 
tive hydrogen  is  repelled.     The  atom 
thus  driven  off  seeks  refuge  with  the 
oxygen  of  the  next  molecule,  and  dis- 
possesses its  hydrogen.     This  atom  in  turn  robs  the 
third  molecule  of  its  oxygen,  and  so  on  until  the  last 


*  It  should  be  noticed  that  the  terms  are  reversed  when  applied 
to  the  plates  and  the  poles.  The  zinc  pole  is  negative,  but  the 
zine  plate  is  positive  ;  the  copper  pole  is  positive,  but  the  copper 
plate  is  negative.  We  thus  see  that  the  plates  when  placed  in 
the  liquid  become  polarized,  as  is  represented  in  the  figure. 

f  In  figure  205,  a  molecule  of  water  is  represented,  for  conven- 
ience, as  consisting  of  only  one  atom  of  hydrogen  and  one  of  oxygen. 


GAL  VANIC  ELECTRICITY.  2^ 

molecule  is  reached,  when  the  atom  of  hydrogen, 
attracted  by  the  negative  copper,  gives  up  to  it  its 
positive  electricity,  and  then  flies  off  into  the  air. 
Each  atom  of  escaping  hydrogen  imparting  its  elec- 
trical force,  adds  to  the  current  of  electricity. 

The  Voltaic  Current. — The  word  "  current"  is  fre- 
quently used  in  electricity,  but  should  not  be  under- 
stood to  indicate  the  passage  of  a  fluid,  like  the  flow 
of  water  in  a  stream,  but  a  mere  transmission  of  the 
electrical  force.  Thus,  if  a  long  pipe  were  perfectly 
filled  with  water,  a  drop  added  at  one  end  would 
thrust  out  a  corresponding  one  at  the  other,  which 
would  not,  however,  be  the  identical  one  dropped 
in,  since  the  force  alone  would  traverse  the  length 
of  the  pipe.  In  the  voltaic  pair  the  current  of  posi- 
tive electricity  sets  out  from  the  positive  zinc 
through  the  liquid  to  the  negative  copper,  thence 
through  the  wire  around  again  to  the  zinc.  If  the 
circuit  is  broken,  the  current  manifests  itself  at  the 
copper  pole.  There  is  also  a  negative  current  pass- 
ing in  the  opposite  direction ;  but,  to  avoid  confusion, 
whenever  the  term  current  is  used,  the  positive  is 
intended. 

In  galvanic  as  in  Motional  electricity,  when  the 
current  passes  through  a  conducting  substance,  as 
a  wire,  rod,  etc.,  the  force  is  transmitted,  not  on  the 
surface,  as  is  sometimes  said,  but  through  the  entire 
thickness  of  the  body.  Each  molecule,  becoming  po- 
larized and  charged,  discharges  its  force  into  the 
next  molecule,  and  so  on.  The  current  thus  moves 


292 


NATURAL  PHILOSOPHY. 


by  a  rapid  succession  of  polarizations  and  dis- 
charges of  the  molecules  of  the  conductor.  With 
what  inconceivable  rapidity  must  these  successive 
changes  take  place  in  an  iron  wire  to  transmit  the 
electric  force,  as  in  recent  experiments,  from  San 
Francisco  to  Boston  and  return  in  one  minute  ! 

A  BATTERY  consists  of  several  voltaic  pairs  so  com- 
bined as  to  increase  the  strength  and  steadiness  of 
the  electric  current. 

SMEE'S  BATTERY. — Each  cell  consists  of  two  plates 
of  zinc  and  one  of  silver  suspended  between  them. 
Fig.  206.  They   are   clamped    together   with 

screws  and  hung  in  a  glass  jar  filled 
with  dilute  sulphuric  acid.  Since 
bubbles  of  hydrogen  gas  tend  to 
collect  on  the  smooth  surface  of  the 
silver  and  interrupt  the  action,  it  is 
roughened  with  finely  divided  plati- 
num. 

GROVE'S  BATTERY  is  what  is  termed 
a  "  two-fluid  battery."     The  outer 
glass  jar  contains  dilute  sulphuric  acid,  in  which  is 
placed  a  hollow  zinc  cylinder  with  a  slit  at  the  side 
Fig.  207.         to  allow  a  free  circulation  of  the  liquid. 
The  inner  cup  is  of  porous  earthenware, 
and  is  filled  with  strong  nitric  acid  (aqua 
fortis),  in  which  is  suspended  a  thin  strip 
of  platinum. 

Chemical  change. — The   water    in   the 
outer   cup   is   decomposed,   the   oxygen 


GA LVAN1C  ELECTRICITY.  393 

uniting  with  the  zinc  and  the  sulphuric  acid  with 
both,  to  make  sulphate  of  zinc.  The  hydrogen,  how- 
ever, does  not  escape,  as  in  Smee's  battery,  but 
passes  into  the  inner  cup  and  tears  apart  the  nitric 
acid,  forming  water  and  nitric  oxide.  The  latter  is 
at  first  absorbed  by  the  liquid,  but  soon  begins  to 
escape  in  corrosive,  blood-red  fumes.  If  the  zinc  is 
properly  amalgamated,  no  action  will  take  place 
while  the  poles  are  separated,  and  the  battery  will 
remain  quiescent,  like  a  sleeping  giant,  but  the  in- 
stant the  wires  are  connected  the  liquid  will  begin 
to  boil  with  the  evolution  of  the  gas,  while  the  elec- 
tric force  will  leap  to  the  poles.  (Rev.  Chem.,  p.  47.) 

Advantages  of  this  Battery. — (1.)  The  hydrogen 
does  not  collect  on  the  negative  (platinum)  plate, 
since  it  is  absorbed  by  the  nitric  acid.  (2.)  The 
liquid  formed  in  the  inner  cup  is  an  excellent  con- 
ductor of  electricity.  (3.)  Platinum  is  a  more  per- 
fect negative  metal  than  copper,  since  it  is  not  acted 
upon  by  the  acid,  and  thus  does  not  tend  to  start  a 
counter-current ;  therefore  platinum  and  zinc  make 
a  better  voltaic  pair  than  copper  and  zinc.  (4.)  The 
additional  decomposition  of  the  nitric  acid  sets  free 
a  great  quantity  of  electricity. 

BUNSEN'S  BATTERY  (Fig.  209)  differs  from  Grove's 
merely  in  substituting  a  carbon  rod  for  the  platinum 
Btrip  in  the  inner  cup.  This,  being  an  excellent 
conductor,  answers  the  same  purpose  and  is  much 
cheaper. 

DANIELL'S  CONSTANT  BATTERY  has  an  outer  copper 


2  94  NA  TURAL  PHIL OSOPHY. 

cup  holding  a  solution  of  blue  vitriol,  and  an  inner 
porous  cup  containing  a  zinc  rod  and  dilute  sul- 
phuric acid.  The  sulphate  of  copper  battery  consists 
of  a  large  zinc  cylinder  suspended  in  a  copper  jar 
containing  a  solution  of  sulphate  of  copper  (blue 
vitriol). 

QUANTITY  AND  INTENSITY. — A  battery  may  develop 
a  great  quantity  of  electricity  having  a  low  degree 
of  intensity,  or  a  small  quantity  having  a  high  in- 
tensity. Thus  a  cup  of  boiling  water  is  intensely 
hot,  while  a  hogshead  full  of  that  which  is  only 
blood-warm  contains  a  great  quantity  of  heat.  The 
intensity  of  the  electric  force  depends  on  the  num- 
ber of  cells  ;  the  quantity,  on  their  size.  An  intensity 
battery  is  formed  by  joining  the  zinc  plate  of  one 
cell  to  the  copper  of  the  next ;  a  quantity  battery, 
by  joining  all  the  zinc  and  all  the  copper  plates  to- 
gether. The  former  method  is  preferable  when 
great  resistance  is  to  be  overcome. 

COMPARISON  OF  FRICTIONAL  WITH  GALVANIC  ELEC- 
TRICITY.— Frictional  electricity  is  noisy,  sudden,  and 
convulsive  ;  galvanic  is  silent,  constant,  and  power- 
ful. The  one  is  a  quick,  violent  blow  ;  the  other  a 
steady,  uniform  pressure.  Intensity  is  the  charac- 
teristic of  the  former,  quantity  of  the  latter.  The 
lightning  will  leap  through  miles  of  intervening  at- 
mosphere, while  the  galvanic  current  will  follow  a 
conductor  around  the  globe,  rather  than  jump  across 
the  gulf  of  a  half  inch  of  air.  The  most  powerful 
frictional  machine  would  be  insufficient  for  tele- 


GAL  VAN1C  ELECTRICITY.  2<)5 

graphing ;  while  despatches  have  been  sent  across 
the  ocean  with  a  tiny  battery  composed  of  "  a  gun- 
cap  and  a  strip  of  zinc,  excited  by  a  drop  of  water 
the  bulk  of  a  tear."  To  decompose  a  grain  of  water 
would  require  over  6,000,000  discharges  from  a  Ley- 
den  jar — enough  electricity  to  charge  a  thunder- 
cloud 35  acres  in  area  ;  yet  a  few  galvanic  cups 
would  tear  apart  that  amount  of  water  in  perfect 
ease  and  silence.  Faraday  immersed  a  voltaic  pair, 
composed  of  a  wire  of  platinum  and  one  of  zinc,  in 
a  solution  of  4  oz.  of  water  and  one  drop  of  oil  of 
vitriol.  In  three  seconds  this  produced  as  great  a 
deviation  of  the  galvanometer  needle  (Fig.  212)  as 
was  obtained  by  30  turns  of  a  powerful  plate-glass 
machine.  If  this  had  been  concentrated  in  one- 
millionth  of  a  second,  the  duration  of  an  electric 
spark,  it  would  have  been  sufficient  to  kill  a  cat ;  yet 
it  would  require  800,000  such  discharges  to  decom- 
pose a  grain  of  water. 

I.  PHYSICAL  EFFECTS  OF  YOLTAIC  ELECTRICITY. — 1. 
Heat. — If  a  current  of  electricity  be  passed  through 
a  wire  too  small  to  conduct  it  readily,  it  is  converted 
into  heat.  The  poorer  the  conducting  power  of  the 
wire,  and  hence  the  greater  the  resistance,  the  more 
readily  the  change  is  produced.  With  10  or  12  of 
Grove's  cups  several  inches  of  fine  steel  wire  may 
be  thus  fused,  or  even  dissipated  into  smoke  ;  and 
with  a  powerful  battery,  several  yards  of  platinum 
wire  (the  poorest  conductor)  may  be  made  glowing 
hot.  Torpedoes  and  blasts  are  fired  on  this  prin- 


296 


NATURAL  PHILOSOPHY. 


ciple.  Two  copper  wires  leading  from  the  battery 
to  the  spot  are  separated  in  the  powder  by  a  short 
piece  of  small  steel  wire.  When  the  circuit  is  com- 
pleted, the  fine  wire  becomes  red  hot  and  explodes 
the  charge. 

2.  Light. — In  closing  or  breaking  the  circuit  we 
produce  a  spark,  the  size  of  which  depends  on  the  in- 
tensity of  the  current.  With  several  cells,  beauti- 

fully  variegated 
sparks  are  obtain- 
ed by  fastening  one 
pole  to  a  file  and 
rubbing  the  other 
upon  it.  When 
charcoal  or  gas- 
carbon  electrodes 
are  used  with  a 
powerful  battery, 
on  slightly  sepa- 
rating the  points, 
the  intervening 
space  will  be  spanned  by  an  arch  of  the  most  dazzling 
light.  The  flame,  reaching  out  from  the  positive 
pole  like  a  tongue,  vibrates  around  the  negative 
pole,  licking  now  on  this  side  and  now  on  that.  The 
heat  is  most  intense.  Platinum  melts  in  it  like  wax 
in  the  flame  of  a  candle,  the  metals  burn  with  their 
characteristic  colors,  and  even  lime,  quartz,  etc.,  are 
fused.  The  effect  is  not  produced  by  burning  the 
charcoal  points,  since  in  a  vacuum  it  is  equally  bril- 


GALVANIC  ELECTRICITY. 


297 


liant.  The  cost  of  the  electric  light  and  the  inten- 
sity of  the  illumination,  which  renders  the  shadows 
extremely  dense,  have  prevented  its  general  use.  It 
is  interesting  to  notice  that  in  the  battery  there  is 
zinc  burning,  i.  e.,  combining  with  oxygen,  but  with- 
out light  or  heat ;  in  the  electric  light  the  real  force 
of  the  combustion  is  revealed.  We  may  thus  trans- 
fer the  light  and  heat  to  a  great  distance  from  the  fire. 
II.  CHEMICAL  EFFECTS. — 1.  Decomposition  of  Wa- 
ter.— If  the  platinum  electrodes  are  held  a  little  dis- 
tance apart  in  a  cup  of  water,  little  trains  of  tiny 

Pig.  209. 


bubbles  will  immediately  begin  to  rise  to  the  sur- 
face. If  the  copper  poles  are  inserted,  bubbles  will 
pass  off  from  the  negative,  but  none  from  the  posi- 
tive pole,  since  the  oxygen  combines  with  the  copper 
wire.  If  the  gases  are  collected,  they  will  be  found 
to  be  oxygen  and  hydrogen,  in  the  proportion  of  two 
parts  of  the  latter  to  one  of  the  former.  The  theory 

13* 


298 


NATURAL  PHILOSOPHY. 


of  the  change  is  the  same  as  that  illustrated  in  Fig. 
205.  It  is  a  curious  fact  that  the  burning  of  an 
atom  of  zinc  in  the  battery  develops  enough  elec- 
tricity to  set  free  an  atom  of  oxygen  at  the  positive 
pole.  This  indicates  a  very  intimate  relation  be- 
tween chemical  affinity  and  electricity — perhaps 
even  their  identity. 

2.  Electrolysis  (to  loosen  by  electricity). — This  is 
the  process  of  the  decomposition  of  compound  bodies 
by  the  voltaic  current.  A  substance  which,  like  wa- 
ter, can  be  separated  in  this  manner,  is  termed  an 
electrolyte. 

Electro-positive  and  Electro-negative  Substances. — In 
the  electrolysis  of  compounds,  their  elements  are 
found  to  be  in  different  electrical  conditions.  Hy- 
drogen and  most  of  the  metals  *  go  to  the  negative 
pole,  and  (since  unlike  electricities  attract)  are  elec- 
tro-positive. Oxygen,  chlorine,  sulphur,  etc.,  go  to 
the  positive  pole,  and  are  therefore  electro-negative. 
In  the  following  list  each  substance  is  electro-nega- 
tive toward  those  which  follow  it,  and  electro-posi- 
tive toward  those  which  precede.  (Berzelius.) 


Electro-negative. 

1.  Oxygen. 

12.  Hydrogen. 

23.  Iron. 

2.  Sulphur. 

13.  Gold. 

24.  Zinc. 

3.  Nitrogen. 

14.  Platinum. 

25.  Manganese. 

4.  Chlorine. 

15.  Mercury. 

26.  Aluminum. 

5.  Iodine. 

16.  Silver. 

27.  Magnesium. 

6.  Phosphorus. 

17.  Copper. 

28.  Calcium. 

7.  Molybdenum. 

18.  Bismuth. 

29.  Barium. 

8.  Tungsten. 

19.  Tin. 

30.  Lithium. 

9.  Carbon. 

20.  Lead. 

31.  Sodium. 

10.  Antimony. 

21.  Cobalt. 

32.  Potass;um. 

11.  Silicon.     . 

22.  Nickel. 

Electro-positive. 

GALVANIC  ELECTRICITY. 


299 


3.  Electro-typing  is  the  process  of  depositing  metals 
from  their  solution  by  means  of  electricity.  It  is 
much  used  in  copying  medals,  woodcuts,  type,  etc. 
An  impression  of  the  object  is  taken  with  gutta- 
percha  or  wax.  The  surface  to  be  copied  is  brushed 
over  with  black-lead  to  render  it  a  conductor.  The 
mould  is  then  suspended  in  a  solution  of  sulphate 
of  copper,  from  the  negative  pole  of  the  battery ;  a 


plate  of  copper  is  hung  opposite  on  the  positive  pole. 
The  electric  current  decomposes  the  sulphate  of 
copper  ;  the  metal  goes  to  the  negative  pole  and  is 
deposited  upon  the  mould,  while  the  acid,  passing  to 
the  positive  pole,  dissolves  the  copper,  and  thus  pre- 
serves the  strength  of  the  solution. 

Duplicates  of  an  engraved  copper-plate  are  pre- 
pared in  the  following  manner.     The  back  of  the 


NATURAL  PHILOSOPHY. 

plate  is  rendered  non-conducting  by  a  coating  of  var- 
nish. The  plate  is  then  suspended  in  the  solution. 
When  the  deposit  of  copper  has  reached  the  re- 
quired thickness,  it  is  stripped  off  without  difficulty. 
This,  of  course,  represents  the  engraved  plate  in 
relief.  If  &fac-simile  is  desired,  a  deposit  is  made 
in  the  same  way  upon  the  copy.  Daguerreotype 
plates  have  been  thus  transferred  without  injury  to 
the  original.  Leaves,  insects,  fruits,  and  even  flow- 
ers, have  been  coated  with  copper  by  this  wonderful 
process. 

While  the  plate  is  hanging  in  the  solution  there 
is  no  noise  heard  or  bubbling  seen.  The  most  deli- 
cate sense  fails  to  detect  any  movement.  Yet  the 
mysterious  electric  force  is  continually  drawing 
particles  of  ruddy,  solid  copper  out  of  the  blue  liquid, 
and,  noiselessly  as  the  fall  of  snowflakes,  dropping 
them  on  the  mould  :  producing  a  metal  purer  than 
any  chemist  can  manufacture,  spreading  it  with  a  uni- 
formity no  artist  can  attain,  and  copying  every  line 
with  a  fidelity  that  knows  no  mistake. 

4.  Electroplating  is  the  process  of  coating  with  silver 
or  gold  by  electricity.  The  metal  is  deposited  most 
readily  on  German  silver,  brass,  copper,  or  nickel  sil- 
ver. The  last  is  a  mixture  of  copper,  zinc,  and  nickel, 
and  is  used  for  the  best  plated- ware.  The  vessels  to  be 
plated  are  thoroughly  cleansed,  and  then  hung  in  a 
solution  of  silver  from  the  negative  pole,  while  a  plate 
of  silver  is  suspended  on  the  positive  pole.  In  five 
minutes  a  mere  "blush"  of  the  metal  will  be  depos- 


G A L  VANIG  ELECT R1C1T  P. 


301 


ited,  which  perfectly  conceals  the  baser  metal  and 
is  susceptible  of  a  high  polish.  It  is  said  that  an 
ounce  of  silver  can  in  this  way  be  spread  over  two 
acres  of  surface.  A  well-plated  spoon  receives  about 
as  much  silver  as  there  is  in  a  ten-cent  piece.  The 
only  method  of  deciding  accurately  the  amount  de- 
posited is  by  weighing  the  article  before  and  after 
being  plated.  A  vessel  is  gold-lined  by  filling  it 
with  a  solution  of  gold,  suspending  in  it  a  slip  of 
gold  from  the  positive  pole  of  the  battery,  and  then 
attaching  the  negative  pole  to  the  vessel.  The  cur- 
rent passing  through  the  liquid  causes  it  to  bubble 
like  soda-water,  and  in  a  few  moments  deposits  a 
thin  film  of  gold  over  the  entire  surface. 

A  simple  Illustration  in  Plating. — Place  in  a  large 
test-tube  a  silver  coin  with  a  little  aqua-fortis.  If 
the  fumes  of  the  decomposed  acid  do  not  soon 
rise,  warm  the  liquid.  When  the  silver  is  dissolved 
fill  the  tube  nearly  full  of  soft  water.  Next  drop 
muriatic  acid  into  the  liquid  until  the  white  precipi- 
tate (chloride  of  silver)  ceases  to  fall.  When  the 
chloride  has  settled,  pour  off  the  colored  water 
which  floats  on  top.  Fill  the  tube  again  with  soft 
water  ;  shake  it  thoroughly  ;  let  it  settle,  and  then 
pour  off  as  before.  Continue  this  process  until  the 
liquid  loses  all  color.  Finally,  fill  with  water  and 
heat  moderately,  adding  cyanide  of  potassium  in 
small  bits  as  it  dissolves,  until  the  chloride  is 
nearly  taken  up.  The  liquid  is  then  ready  for  elec- 
tro-plating. Thoroughly  cleanse  a  brass  key,  hang 


302 


NATURAL  PHILOSOPHY. 


it  from  the  negative  pole  of  a  small  battery  and 
suspend  a  silver  coin  from  the  positive  pole.  Place 
these  in  the  silver  solution,  very  near  and  facing 
each  other.  When  well  whitened  by  the  deposit  of 
silver,  remove  the  key  and  polish  it  with  chalk.  In 
the  arts  the  polishing  is  performed  by  rubbing  with 
"  burnishers."  These  are  made  of  polished  steel, 
and  fit  the  surfaces  of  the  various  articles  upon 
which  they  are  to  be  used. 

III.  PHYSIOLOGICAL  EFFECTS. — With  a  single  cell 
no  effect  ^s  experienced  when  the  two  poles  are  held 
in  the  hands.  With  a  large  battery  a  sudden  twinge 
is  felt,  and  the  shock  becomes  painful  and  even  dan- 
gerous, especially  if  the  palms  are  moistened  with 
salt-water,  which  increases  the  conduction.  Bab- 
bits which  had  been  suffocated  for  half  an  hour, 
have  been  restored  to  life  by  an  application  of  the 
galvanic  current. 


ELECTBO-MAGNETISM. 

EFFECT  OF  A  YOLTAIC  CURRENT  ON  A  MAGNETIC 
NEEDLE. — If  a  current  of  electricity  be  passed  over 
a  magnetic  needle,  the  needle  will  turn  and  tend  to 
place  itself  at  right  angles  to  the  wire.  If  the  wire 
be  brought  over  and  beneath  the  needle,  it  doubles 
the  effect,  and  the  play  of  the  needle  becomes  a  very 
delicate  test  of  the  presence  and  direction  of  the 
electric  force. 


ELECTR  0-MA  GNETISM. 
Fig.  211. 


303 


THE  GALVANOMETER  is  an  instrument  for  measur- 
ing the  force  and  direction  of  an  electric  current.     B 

Fig,  212. 


9O4  NATURAL  PHILOSOPHY. 

is  a  coil  of  wire,  wound  with  thread  to  insulate  it  and 
compel  the  electricity  to  pass  through  the  whole 
length  ;  the  current  is  represented  as  entering  at  n 
and  leaving  at  m.  The  silk  cord,  s,  supports  an 
astatic  needle.  This  consists  of  two  magnetic  nee- 
dles, one  over  the  graduated  circle  and  the  other 
within  the  coil,  with  the  north  pole  of  the  one  oppo- 
site the  south  pole  of  the  other,  so  as  to  neutralize 
the  attraction  of  the  earth,  and  permit  the  combined 
needle  to  obey  the  will  of  the  current  alone.  This 
affords  a  means  of  testing  the  faintest  flow  of 
electricity. 

ELECTRO-MAGNETS. — The  voltaic  current  produces 
magnetism.  If  a  current  be  passed  through  the 
wire  shown  in  Fig.  201,  the  steel  bar  will  be  rendered 

magnetic.     This  shows  the 

f.  213. 

identity  of  the  electricity 
from  the  voltaic  battery  with 
that  from  the  Ley  den  jar.  If 
the  wire  be  wound  around 
a  bar  of  soft  iron,  as  in  Fig. 
213,  the  iron  will  instantly 
become  a  magnet  which 
will  grasp  the  armature 
with  great  force,  but  will  as 
quickly  lose  its  properties 
when  the  current  is  broken.  Electro-magnets  have 
been  made  that  would  lift  3,500  times  their  own 
weight.  If  the  current  be  passed  through  a  coil  of 
insulated  wire  (a  helix),  as  in  Fig.  214,  a  rod 


ELEGTR  0-MA  GNETISM. 


305 


of  iron,  when  held  below  it,  will 
be  drawn  up  into  it  forcibly,  as 
if  pulled  by  a  powerful  spring ; 
thus  realizing  in  science  the  fabu- 
lous story  of  Mahomet's  coffin,  which 
is  said  to  have  been  suspended  in 
mid-air.  Here  we  see  that  not  only 
does  the  soft  iron  within  become 
magnetic,  but  also  the  coil  itself. 
Bar-magnets  are  now  made  by  in- 
serting them  in  a  large  coil  through 
which  a  powerful  current  is  pass- 
ing. 

Motion  produced  by  Electricity. — If 
we  reverse  the  direction  of  the  cur- 
rent, it  changes  the  poles  of  the 
magnet.     Advantage  is  taken  of  this  principle  in 
order  to  produce  continuous  motion.     Fig.  215  rep- 
resents Page's  rotating  machine. 
It  consists  of  an  upright  horse- 
shoe magnet,  between  the  poles 
of  which  is  a  small  electro-mag- 
net.    Above  this  are  two  springs, 
which  are  so  placed  that,  as  the 
central   rod    revolves    with    the 
electro-magnet,  the  current  passes 
through  these  springs,  alternate- 
ly, to  the  wire  coiled  about  the 
iron  of  the  electro-magnet.     The 
poles  of  the  electro-magnet  are 


Fig.  215. 


306 


NATURAL  PHILOSOPHY. 


thus  changed  twice  with  each  revolution.  The  poles 
of  the  upright  magnet  attract  the  opposite  poles  of 
the  electro- magnet,  but  as  soon  as  they  face  each 
other  the  current  is  reversed,  and  they  at  once  repel 
each  other  :  the  other  poles  are  now  attracted,  but  as 
they  come  together  are  repelled  as  before.  A  rapid 
motion  is  thus  secured.  The  revolutions  may  rise 
as  high  as  2,500,  making  5,000  reversals  of  the  cur- 
rent in  a  minute. 

Electro-magnetic  engines  are  constructed  either  on 
the  principle  that  the  magnet  retains  its  power  only 
while  the  current  is  passing,  or  that  the  poles  are 
changed  by  reversing  the  current.  They  have  been 
made  of  8  or  10  horse-power,  yet  have  never  become 
of  great  practical  value,  because  of  the  expense  of 
the  battery  required  to  produce  the  electricity.  The 
zinc  which  burns  in  the  cell  of  the  electric-engine 
is  far  more  expensive  than  the  coal  which  burns  in 
the  furnace  of  the*  steam-engine. 

The  Electro-magnetic  Telegraph  depends  on  the 
same  principle  as  the  electro-magnet.  A  single 
wire  is  used  to  connect  the  two  stations  between 
which  despatches  are  to  be  sent.  The  extremi- 
ties of  the  wire  extend  into  the  ground,  and  the 
earth  completes  the  circuit.  Each  station  has  a 
key  and  a  receiver  •  the  former  is  used  for  sending 
messages,  and  the  latter  for  receiving  them.  The 
key  is  shown  in  Fig.  216.  The  wire,  P,  leads  from 
the  battery  ;  L  is  the  line-wire,  and  A  connects  with 
the  receiver  ;  a  brass  lever,  h  k,  turns  on  an  axis. 


ELECTR  0-MA  GNETISM.  o  o  y 

A  spring,  r,  elevates  the  lever,  and  keeps  the  pin,  a, 
pressed  down  upon  a  little  button  just  below,  to 
which  the  wire,  Fig.  210. 

A,  is  attached. 
The  key  is  now  in 
a  condition  to  re- 
ceive a  message. 
The  current  from 
L  passes  through 
the  lever  k  down  the  pin  a,  along  the  wire  A,  to  the  re- 
cording instrument,  and  thence  to  the  earth,  making 
the  circuit  complete.  To  send  a  message,  the  but- 
ton B  is  pressed  down  by  the  finger  of  the  operator, 
so  as  to  strike  the  button  below  it ;  the  circuit  is 
established  there  and  broken  beneath  a.  The  cur- 
rent from  the  battery  at  the  station  now  passes  from 
P  through  li  to  L.  The  operator,  by  elevating  or 
depressing  B,  can  thus  break  or  complete  the  circuit 
at  his  option.  At  the  station  whfere  the  despatch  is 
received,  the  current  passes,  as  we  have  seen,  di- 
rectly into  the  receiver.  This  contains  an  electro- 
magnet, E.  When  the  circuit  is  complete,  the  cur- 
rent, flashing  through  the  coils  of  wire  at  E,  at- 
tracts the  armature,  m.  This  elevates  n,  the  other 
end  of  the  lever,  m  n,  and  forces  the  sharp  point,  x, 
firmly  against  the  soft  paper,  a.  As  soon  as  the 
circuit  is  broken,  E  ceases  to  be  a  magnet,  and  the 
spring,  R,  lifts  the  armature,  drawing  the  point  from 
the  paper.  Clock-work  attached  to  the  rollers  at  z 
moves  the  paper  along  uniformly  beneath  the  point 


NATURAL  PHILOSOPHY. 
Fig.  217. 


x.  When  the  circuit  is  completed  and  broken  in- 
stantly, there  is  a  sharp  dot  made  on  the  paper. 
This  is  called  e ;  two  dots,  i ;  three  dots,  s ;  four 
dots,  h.  If  the  current  is  closed  for  a  longer  time, 
the  mark  becomes  a  dash  ;  this  is  t ;  two  dashes, 
m  ;  a  dot  and  a  dash,  a. 

TABLE  OF  MOBSE'S  SIGNS, 


a  .  -- 

j     .  

s    ... 

b  

k  

t    _ 

C    .                               i 

9         1 

n   >  t  

d  

m  

V     .  .  

e  . 

n   — 

W   .  

f     

o    .     . 

X     .  . 

or 

v 

s  — 

ll   

P. 
q    

J 

z    .  .  .    . 

i    .  . 

r    .     .  . 

&  .... 

A  skilful  operator  becomes  so  used  to  the  sound 
that  the  clicking  of  the  armature  is  perfectly  intel- 
ligible. He  uses,  therefore,  simply  a  "  sounder"  i.  e., 
a  receiver  without  the  paper  and  clock-work  attach- 
mentr  We  thus  see  that  the  principle  of  the  tele* 


ELECTS  0-MA  GNETISM.  ^Of) 

graph  consists  in  dosing  and  breaking  the  circuit  at  one 
station,  and  in  making  and  unmaking  an  electro-magnet 
at  the  other. 

The  Eelay. — When  the  stations  are  more  than  fifty 
miles  apart,  the  current  becomes  too  weak  to  work 
the  receiver.  The  relay  uses  the  force  of  a  local 

Fig.  2ia 


battery  for  this  purpose.  L  is  the  line-wire  ;  T  the 
ground-wire  ;  c  is  connected  with  the  positive  pole  ; 
Z  with  the  receiver,  and  thence  with  the  negative 
pole  of  the  battery.  The  corrupt  passes  in  at  L, 
traverses  the  fine  wire  of  the  electro-magnet,  E,  and 
thence  passes  out  at  T  to  the  ground.  The  arma- 
ture A,  playing  to  and  fro  as  the  current  from  the 
distant  station  darts  through  or  is  cut  off,  moves  the 
lever  p,  which  works  on  an  axis  at  its  centre,  and  is 
drawn  back  by  the  spring  r.  As  A  is  attracted,  p 
strikes  against  the  screw  n  ;  the  current  from  C  leaps 
up  m  to  n,  down  p  and  through  Z  to  the  electro- 
magnet of  the  receiver  and  attracts  its  armature. 
The  operator  who  sends  the  message  simply  com- 
pletes and  breaks  the  circuit  with  the  key,  the  ar- 


3io 


NATURAL  PHILOSOPHY. 


mature  of  the  relay ,at  the  station  where  the  message 
is  received,  vibrates  in  unison  with  these  movements, 
the  receiver  or  sounder  repeats  them  with  greater 
force,  and  the  second  operator  interprets  their 
meaning. 

MAGNETO-ELECTRICITY  is  that  which  is  developed 
by  means  of  magnetism.  A  common  form  of  a  ma- 
chine for  this  purpose  is  shown  in  Fig.  219.  Coils 

Fig.  219. 


of  wire  are  carefully  insulated  and  wound  around  a 
small  bar  of  soft  iron,  B,  bent  at  right  angles.  This 
acts  as  the  armature  of  a  powerful  horse-shoe  mag- 
net, before  the  poles  of  which  it  is  made  to  revolve. 
The  soft  iron  becomes  magnetic,  and  then  induces 
electric  currents  in  the  coils.  The  poles  are  changed 
twice,  and  thus  two  opposite  currents  are  induced 
in  each  revolution.  By  means  of  a  break-piece 
the  circuit  is  rapidly  broken  and  closed.  Severe 


ELECTE  0-MA  GNETISM.  ^  l  l 

shocks  are  thus  produced,  when  the  poles  are  grasped 
by  the  hands.* 

In  Wilde's  machine,  the  induced  current  from  the 
coils  is  carried  around  a  large  electro-magnet,  which 
is  thereby  excited  to  a  high  degree.  The  armature 
revolving  before  this  furnishes  the  current  which  is 
used.  A  machine  lately  exhibited  was  driven  by  a 
steam-engine  of  7-horse  power.  The  poles  were  wire- 
rope,  a  quarter  of  an  inch  in  diameter  and  140  feet 
long.  It  produced  an  electric  light  dazzling  as  the 
noonday  sun,  throwing  the  flame  of  the  street-lamps 
into  shade  at  a  quajter-mile  distance.  Its  heat  was 
sufficient  to  fuse  a  rod  of  iron  a  quarter  of  an  inch 
in.  diameter  and  fourteen  inches  long,  and  could  be 
felt  fifty  yards  away.  When  one  pole  was  inserted  in 
the  canal,  and  the  other  in  a  pool  two  hundred  feet 
distant,  the  water  was  decomposed,  oxygen  gas  bub- 
bling up  at  one  electrode,  and  hydrogen  at  the 
other. 

INDUCED  CURRENTS. — Let  two  coils  of  wire  be  made 
to  fit  into  each  other,  and  carefully  separated  by 
insulators.  If  a  current  of  electricity  be  passed 

*  A  Yankee  once  threw  the  industrial  world  of  Europe  into  a 
wonderful  excitement  by  announcing  a  new  theoiy  of  perpetual 
motion  based  on  the  magneto-electric  machine.  He  proposed  to 
decompose  water  by  the  current  of  electricity,  then  bum  the  hy- 
drogen and  oxygen  thus  obtained.  In  this  way  he  would  drive 
a  small  steam-engine,  which,  in  turn,  would  keep  the  magneto- 
electric  machine  in  motion.  This  would  certainly  be  a  splendid 
discoveiy.  It  would  be  a  steam-engine  which  would  prepare  its 
own  fuel,  and,  in  addition,  dispense  light  and  heat  to  all  around, 
(Helmholtz.) 


312 


NATURAL  PHILOSOPHY. 


through  the  inner  coil,  it  will  induce  a  powerful 
secondary  current,  flowing  in  the  opposite  direction^ 
in  the  outer  coil.  This  soon  ceases  :  on  breaking 
the  circuit,  however,  it  will  start  again,  but  in  the 
same  direction  as  the  primary  current.  The  appa- 
ratus shown  in  Fig.  220  consists  essentially  of  the 

Pig.  220. 


two  coils  just  described.  The  primary  current  from 
a  single  cell  is  rapidly  interrupted  by  means  of  a 
small  electro-magnet.  When  this  is  magnetized,  it 
attracts  the  armature,  and  thus  the  circuit  is  broken ; 
the  armature  immediately  springs  back,  and  again 
completes  the  circuit.  A  bunch  of  iron  wires  may 
be  inserted  as  a  core  in  the  inner  coil.  When  the 
current  passes,  these  become  magnetized,  and  by 
induction  largely  strengthen  the  secondary  current. 
This  form  is  much  used  for  medical  purposes. 
Ruhmkorffs  coil  is  constructed  on  the  same  prin- 
ciple. The  largest  coils  often  contain  thirty  to  fifty 
miles  of  covered  wire.  Kitchie,  of  Boston,  has  de- 
vised many  ingenious  improvements  which  render 
the  current  extremely  intense.  »His  15-inch  coils 
will  throw  a  quick  succession  of  sparks,  each  fifteen 


THERMAL  ELECTRICITY.  3  1 3 

inches  long,  charge  and  discharge  a  Ley  den  jar,  with 
a  crack  like  that  of  a  pistol,  as  rapidly  as  one  can 
count,  and  perform  the  vacuum  experiments  in 
frictional  electricity  with  a  splendor  and  brilliancy 
no  plate-machine  can  rival. 

THEEMAL  ELECTRICITY. 

As  electricity  can  be  changed  into  heat,  in  turn 
heat  can  be  converted  into  electricity.  A  Thermo- 
electric pile  consists  of  alternate  bars  of  antimony 
and  bismuth  soldered  together,  as  shown  in  Fig.  222. 
When  mounted  for  Fig.  221.  Fig.  222. 

use,  the  couples  are 
insulated  from  each 
other  and  enclosed 
in  a  copper  frame  P. 
If  both  faces  of  the 
pile  are  equally 
heated,  there  is  no 
current.  The  least  variation  of  temperature,  how- 
ever, between  the  two  is  indicated  by  the  flow  of 
electricity.  Wires  from  a,  the  positive  pole,  and  6, 
the  negative,  connect  the  pile  with  the  galvanometer 
(Fig.  212).  This  constitutes  one  of  the  most  delicate 
tests  of  the  presence  of  heat.  A  tiny  insect  held 
against  the  face  of  the  pile  will  move  the  needle.' 
Strange,  that  minute  quantities  of  heat  become  sen- 
sible only  when  tfiey  are  converted  into  electricity, 
then  into  magnetism,  and  lastly  into  motion  ! 


,  j  4  XA  TUli A  L  PHIL  OS  OP11  \ '. 

ANIMAL  ELECTEICITY. 

ELECTRIC  FISH  have  the  property  of  giving,  when 
touched,  a  shock  like  that  from  a  Ley  den  jar. 
The  torpedo  and  electrical  eel  are  the  most  noted. 
The  former  is  a  native  of  the  Mediterranean,  and 
its  shock  was  anciently  much  prized  as  a  cure  for 
various  diseases.  The  latter  is  abundant  in  certain 
South  American  waters.  A  specimen  of  this  fish, 
forty  inches  in  length,  was  estimated  by  Faraday  to 
emit  a  spark  equal  to  the  discharge  of  a  battery  of 
fifteen  Leyden  jars.  The  Indians  are  said  to  be  ac- 
customed to  drive  herds  of  wild  horses  into  the 
streams  frequented  by  the  fish.  The  horses  are  soon 
overpowered  by  the  terrible  shocks  they  receive, 
and  so  fall  an  easy  prey  to  their  pursuers. 


CONCLUSION. 

"  Science  is  a  psalm  and  a  prayer."— PAKKER. 

NOWHERE  in  nature  do  we  find  chance.  Every 
event  is  governed  by  fixed  laws.  If  we  would  ac- 
complish any  result  or  perform  any  experiment,  we 
must  come  into  exact  harmony  with  the  universal 
system.  If  we  deviate  from  the  line  of  law  by  a 
hair's  breadth,  we  fail.  These  laws  have  been  in 
operation  since  the  creation,  and  all  the  discoveries 
of  science  prove  them  to  extend  to  the  most  distant 


CONCLUSION. 


315 


star  in  space.  A  child  of  to-day  amuses  itself  with 
casting  a  stone  into  the  brook  and  watching  the  wi- 
dening curves  :  little  antediluvian  children  could 
have  done  the  same.  A  law  of  nature  has  no  force 
of  itself ;  it  is  but  the  manner  in  which  force  acts. 
We  cannot  create  force.  We  can  only  take  it  as  a 
gift  from  God.  We  find  it  everywhere  in  Nature. 
Matter  is  not  dumb,  but  full  of  inherent  energy.  A 
tiny  drop  of  dew  sparkling  on  a  spire  of  grass  is  in- 
stinct with  power :  Gravity  draws  it  to  the  earth ; 
Chemical  Affinity  binds  together  the  atoms  of  hydro- 
gen and  oxygen  *,  Cohesion  holds  the  molecules  of 
water,  and  gathers  the  drop  into  a  globe ;  Heat  keeps 
it  in  the  liquid  form  ;  Adhesion  causes  it  to  cling  to 
the  leaf.  If  the  water  be  decomposed,  Electricity 
would  be  set  free  ;  and  from  this,  Heat,  Light,  Mag- 
netism, and  Motion  could  be  produced.  Thus  the 
commonest  object  becomes  full  of  fascination  to  the 
scientific  mind,  since  in  it  reside  the  mysterious 
forces  of  Nature. 

These  various  forces  can  be  classified  either  as 
attractive  or  repeUant.  Under  their  influence  the 
atoms  or  molecules  resemble  little  magnets  with 
positive  and  negative  poles.  They  therefore  ap- 
proach or  recede  from  each  other,  and  so  tend  to 
arrange  themselves  according  to  some  definite  plan. 
"  The  atoms  march  in  time,  moving  to  the  music  of 
law."  A  crystal  is  but  a  specimen  of  "  molecular 
architecture"  built  up  by  the  forces  with  which  mat- 
ter is  endowed. 


3-6 


NATURAL  PHILOSOPHY. 


No  force  can  be  destroyed.  A  hammer  falls  by 
the  force  of  gravity  and  comes  to  rest,  but  its  mo- 
tion as  a  mass  is  converted  into  a  motion  of  atoms, 
and  reveals  itself  to  the  sense  of  touch  as  heat. 
Thus  force  changes  its  form  continually,  but  the  eye 
of  philosophy  detects  it  and  enables  us  to  drive  it 
from  its  various  hiding-places  still  undiminished.  It 
assumes  Protean  guises,  but  is  doubtless  essentially 
a  unit  everywhere.  It  may  disappear  from  the 
earth  ;  still — 

"  Somewhere  yet  that  atom's  force 
Moves  the  light  poised  universe." 

This  conversion  of  force  is  termed  the  "  CORRELATION 
OP  THE  PHYSICAL  FORCES."  It  is  the  grandest  law  Na- 
ture offers  for  the  contemplation  of  the  human  mind. 
What  is  the  nature  of  force  we  cannot  tell.  We 
think  it  to  be  a  mode  of  motion.  Beyond  this,  all 
is  mystery. 

The  forces  of  Nature  are  strangely  linked  with  our 
lives.  Everywhere  a  Divine  Hand  is  developing 
ideas  tenderly  and  wondrously  related  to  human 
needs.  To  the  thoughtful  mind  all  phenomena  have 
a  hidden  meaning. 

"  To  matter  or  to  force 

The  all  is  not  confined ; 

Beside  the  law  of  things 

Is  set  the  law  of  mind ; 

One  speaks  in  rock  and  star, 

And  one  within  the  brain, 

In  unison  at  times, 

And  then  apart  again. 
And  both  in  one  have  brought  us  hither 
That  we  may  know  our  whence  and  whithei 


CONCLUSION. 

"  The  sequences  of  law 
We  learn  through  mind  alone ; 
We  see  but  outward  forms, 
The  soul  the  one  thing  known  ; — 
If  she  speak  truth  at  all, 
The  voices  must  be  true 
That  give  these  visible  things, 
These  laws,  their  honor  due, 
But  tell  of  One  who  brought  us  hither 
And  holds  the  keys  of  whence  and  whither. 


"  He  in  His  science  plans 
What  no  known  laws  foretell ; 
The  wandering  fires  and  fixed 
Alike  are  miracle : 
The  common  death  of  all, 
The  life  renewed  above, 
Are  both  within  the  scheme 
Of  that  all-circling  love. 
The  seeming  chance  that  cast  us  hither 
Accomplishes  His  whence  and  whither.- 


317 


NATIONAL  SCHOOL  APPARATUS, 


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NOTES 

ON  APPARATUS  AND  EXPERIMENTS. 


PAGE 

30.  An  Ivory  ball  from  apparatus  Fig.  33  can  be  used  for 
this  experiment. 

40.  Half-dozen  Rupert's  Drops.     A  glass  funnel,  pack  of  fil- 

ters, and  i  Ib.  animal  charcoal. 

41.  I  Ib.  soft  French  glass  tubing,  assorted  sizes.     A  4  02. 

alcohol  lamp  is  also  necessary. 

44.  Instead  of  the  blue  litmus,  a  solution  of  cabbage  is  good 
in  this  experiment.  It  is  made  by  steeping  purple 
cabbage-leaves  in  water,  until  the  colored  juice  is  ex- 
tracted. The  funnel  can  be  made  of  tin,  by  any  tin- 
smith. 

46.  A  Grove's  cup,  fitted  with  cork  and  tube.  This  may  be 
supported  with  a  wire  tripod  or  any  convenient  device. 
The  hydrogen  preparation  is  described  in  Chemistry. 

51.  Long  tube  for  Guinea  and  Feather  experiment.  It  may 
also  be  used  to  perform  the  experiment  on  page  282. 

54.  Fig.  14  can  be  easily  made.    The  rocking-horse,  Fig.  if. 
illustrates  the  principle  more  forcibly. 

58.  Set  of  Pendulums,  Fig.  19. 

59.  This  apparatus  is  not  as  essential  as  the  last  named, 

though  very  useful. 

60.  The  apparatus,  Fig.  21,  can  be  made  by  any  carpenter. 

The  pendulums  are  turned  from  hard  wood,  and  hung 

on  wire- hooks. 

71.  Apparatus  to  explain  2d  law  of  motion. 
77   Centrifugal-force  apparatus,  Fig.  32. 


}20  NOTES   ON   APPARATUS  AND    EXPERIMENTS. 

79.  Action  and  Reaction  apparatus,  Fig.  33. 

85.  Model  of  the  Mechanical  Powers. 

104.  Half-dozen  tubes  with  bulbs,  as  in  Fig.  67. 

106.  Model  of  Hydrostatic  Press. 

1 08.  This  series  of  tubes  can  be  easily  made  by  any  teacher 

having  the  glass  tubing  and  a  spirit-lamp. 

109.  Apparatus  shown  in  Fig.  72. 

1 10.  Apparatus  shown  in  Fig.  73  or  74.     The  former  is  less 

liable  to  be  injured  by  use. 

117.  Hydrostatic  balance  and  weights.      This  instrument  is 

furnished  with  glass  and  brass  disks  to  estimate  the 
adhesion  of  solids  and  liquids. 

1 1 8.  Hydrometer  and  jar.    The  jar  may  also  be  used  in  Figs.  8 

and  83. 

128.   Model  of  Barker's  Mill. 

133.  A  table  air-pump.  This  is  the  best  and  cheapest  form 
of  the  air-pump.  A  barometer- gauge  is  a  valuable 
addition.  A  condensing  air-chamber,  syringe,  and 
jets,  form  a  most  valuable  counterpart  of  the  air- 
pump.  By  means  of  it,  many  instructive  experiments 
in  Hydraulics  and  Pneumatics  may  be  performed. 

133.  A  copper  flask  and  stop-cock,  Fig.  93. 

134.  Two  black  Cartesian  imps. 

136.  Hand-glass.     Magdeburg  Hemispheres. 

137.  Upward-pressure  apparatus. 

138.  Apparatus  shown  in  Fig.  101. 

138.  Barometer-tube,  open  at  both  ends.   .3  Ibs.  of  mercury. 

142.  Model  of  forcing  and  lifting  pumps. 

146.  A  glass  siphon,  with  tube  for  exhausting  the  air. 

155.  Sound  in  vacuo.  A  much  cheaper  apparatus  than  the 
one  shown  in  the  figure  will  answer  the  purpose  of  this 
experiment.  The  bell  may  be  suspended  by  a  cord 
and  rung  by  a  sliding  rod,  or  by  simply  tilting  the 
puinp.  The  effect  will  not  be  as  complete  as  thai 
stated  in  the  text. 

175.  Figs.  123  ana  125  can  De  made  by  any  ingenious  pupil, 
and  will  afford  profitable  anrixntent.  Try  them. 


NOTES   ON    APPARATUS   AND   EXPERIMENTS.  32 1 

173.  A  vibrating-plate  and  violin-bow,  Fig.  125. 

183.  Glass  tubes  for  singing  flames.  The  experiment  is  most 
satisfactory  when  an  apparatus  like  that  shown  in  the 
figure  is  employed.  The  tube  may,  however,  be  held 
by  the  hand.  The  beaks  of  broken  retorts  make  ex- 
cellent tubes. 

194,  A  concave  and  convex  mirror  in  one  frame. 

200.  A  set  of  small  lenses. 

201.  A  large  double  convex  lens,  mounted. 

^O5  Mounted  prism.  The  lens  just  mentioned  can  be  used 
for  the  recomposition  of  the  light,  but  a  more  striking 
way  is  to  use  a  painted  disk,  to  be  attached  to  the  ap- 
paratus for  centrifugal  force,  Fig.  32. 

215    A  compound  microscope,  with  mounted  objects. 

219.  Magic  lantern.  This  is  capable  of  almost  unlimited  use, 
if  means  can  be  procured  to  purchase  mounted  slides 
illustrative  of  principles  in  Astronomy,  Geology,  Bot- 
any, Physiology,  etc. 

234.  A  compound  bar  to  illustrate  unequal  expansion  of  metals. 

240.  A  Florence  Flask,  Fig.  171.  This  flask  may  be  used 
also  for  Fig.  174. 

240.  Water-hammer  or  Pulse-glass. 

261.  Bar  and  horse-shoe  magnets.     I  Ib.  iron-filings. 

263.  Small  horizontal  and  dipping  needles. 

273.  Electrical  machine,  electric  whirl,  and  brass  chain.     An 

insulating  stool  may  be  extemporized  with  an  ordinary 
stool,  by  setting  the  legs  in  glass  tumblers. 

274.  Small  insulated  conductor. 

276.  Electric  chime. 

277.  Dancing-image  plates,  and  pith-ball  image. 

278.  Leyden  jar  with  movable  coatings. 
277.  Leyden  jar  and  discharger. 

286.  Spiral  tube  and  diamond  Leyden  jar. 

(All  the  experiments  in  Galvanism  and  Electro-magnet- 
ism can  be  performed  with  a  large  sulphate  of  copper 
battery,  except  the  decomposition  of  water  and  the 
electric  light.  This  battery  is  cheap,  and  very  con- 


J22  NOTES   ON    APPARATUS   AND    EXPERIMENTS. 

venient  to  use.  For  the  other  experiments  a  battery 
of  5  to  12  of  Grove's  Cups  will  answer,  though  the 
electric  arch  cannot  be  well  exhibited  with  less  than 
40  to  60  cups. 

304.  An  electro-magnet. 

^05.    A  lifting  coil. 

',05.   Page's  rotating  machine. 

507-9.   Model  of  a  telegraphic  machine. 

310.    Magneto-electric  machine  in  a  box. 

312.  Electro-magnetic  machine,  Fig.  220. 

313.  Thermo-electric  pile  and  galvanometer,  Figs.  221  &  222, 


CF"  Priced  tists  of  the  above  apparatus  will  be  Jurnishea 
Oft  application  to  A.  S.  BARNES  &*  CO., 

in  and  113  William  St.,  New  Yorl, 


QUESTIONS. 


THE  following  questions  are  those  which  the  author  has 
used  in  his  own  classes,  both  as  a  daily  review  and  in  examina- 
tion. A  standing  question,  which  has  followed  every  other 
question,  has  been:  "  Can  you  illustrate  this?"  Without, 
therefore,  a  particular  request,  the  pupil  has  been  accustomed 
to  give  as  many  practical  examples  as  he  could,  whenever  he 
has  made  any  statement  or  given  any  definition.  The  figures 
refer  to  the  page  of  the  book. 

INTRODUCTION. — Define  matter.  A  body.  A  substance. 
Name  and  define  the  two  kinds  of  properties  which  belong  to 
each  substance. 

14.  The  two  kinds  of  change.     What  is  the  principal  dis- 
tinction between  Philosophy  and  Chemistry  ?     Mention  some 
phenomena  which  belong  to  each.     Why  are  these  branches 
intimately  related  ? 

15.  Name  the  general  properties  of  matter.     Define  magni- 
tude.    Size.     Why  is  it  necessary  to  have  a  standard  of  meas- 
ure ?     Whence  were  the  ancient  standards  derived  ?     Give  the 
history  of  the  English  standard. 

1 6.  Is  the  American  yard  an  exact  copy  of  the  English  ? 
Have  vve  any  national  standard  ?      Give  an  account  of   the 
French  system.      By  what  name   is  this  system   commonly 
known  ?     Is  either  of  these  systems  founded  on  any  natural 
standard  ?     Why  is  it  desirable  to  have  such  a  standard  ? 

17.  Define  Impenetrability.      Give  some  apparent  excep- 
tions, and  explain  them.     Define  Divisibility. 

19.  Is  there  any  limit  to  the  divisibility  of  matter  ?     Explain 
the  Atomic  Theory.     What  use  has  it  ? 

20.  How  do  animalcule  illustrate  this  subject?     Under  a 
powerful  microscope  how  would  chalk-marks  appear  ? 

21.  Define  Porosity.     Is  the  word  porous  here  used  in  its 
common  acceptation ?    Define  a  molecule.     An  atom.     Com- 
pare the  size  of  an  atom  with  that  of  a  pore. 


324  QUESTIONS  IN  PHILOSOPHY. 

25.  Define  Inertia.     Does  a  ball,  when  thrown,  stop  itself? 
Why  is  it  difficult  to  start  a  heavy  wagon  ?     Why  is  it  danger- 
ous to  jump  from  the  cars  when  in  motion  ? 

26.  Define  Indestructibility.      Did  the  earth,  at  its  creation, 
contain  the  same  quantity  of  matter  it  does  now? 

27.  Name  the  specific  properties  of  matter.     Define  Duc- 
tility.    How  is  iron  wire  made? 

28.  Platinum  wire?    Gilt  wire?    What  is  said  of  brass  wire  ? 
Define  Malleability.     Describe  the  manufacture  of  gold-leaf. 

29.  Is  copper  malleable  ?     Define  Tenacity.      Name  and 
define  the  three  kinds  of  Elasticity.      Illustrate  the  elasticity 
of  compression  as  seen  in  solids. 

30.  In  liquids.     In  gases.     What  is  said  about  the  relative 
compressibility  of  liquids  and  gases  ?     Compare  air  with  water. 

31.  Illustrate  the  elasticity  of  expansion  as  seen  in  solids, 
liquids,  and  gases.     Define  Elasticity  of  Torsion.     What  is  a 
Torsion  balance  ?     Define  Hardness.     Does  this  property  de- 
pend on  density? 

32.  Define  Density.      Define  Brittleness.      Is  a  hard  body 
necessarily  brittle  ?     Why  are  feathers  light  and  lead  heavy? 

MOLECULAR  FORCES. — Define  a  molecular  force.  What 
two  opposing  forces  act  between  the  molecules  of  matter  ? 
How  is  this  shown  ?  What  is  the  repellant  force  ?  Name  the 
attractive  forces.  Which  of  these  belong  to  Philosophy? 

COHESION. — Define.  What  are  the  three  states  of  matter? 
Define.  How  can  a  body  be  changed  from  one  state  to  an- 
other ?  Show  that  cohesion  acts  only  at  insensible  distances. 
Explain  the  process  of  welding. 

37.  Why  cannot  all  metals  be  welded  ?     Why  do  drops  of 
dew,  etc.,  take  a  globular  form?     Why  do  not  all  bodies  have 
this  form  ? 

38.  Illustrate  the  tendency  of  matter  to  a  ciystalline  struc- 
ture.    Has  each  substance  its  own  form? 

39.  Why  is  not  cast-iron  crystalline  ?     Why  do  the  axles  of 
cars  become  brittle  after  use  ?     Describe  the  process  of  tem- 
pering and  annealing. 

40.  Explain  the  Rupert  Drop. 

ADHESION. — Define.  What  is  the  theory  of  filtering  through 
charcoal  ? 

41.  Of  what  use  is  soap  in  making  bubbles?     Define  Capil- 
lary Attraction.     Why  will  water  rise  in  a  glass  tube,  while 
mercury  will  be   depressed  ?      Is  a  tube  necessary  to  show 
capillary  attraction  ?    What  is  the  law  of  the  rise  in  tubes  ? 


QUESTIONS  IN  PHILOSOPHY. 


325 


42-3.  Give  practical  illustrations  of  capillary  action.  Why 
will  not  old  cloth  shrink  as  well  as  new,  when  washed  ? 

44.  What  is  the  cause  of  solution  ?  Why  is  the  process 
hastened  by  pulverizing  ?  Tell  what  you  can  about  gases  dis- 
solving in  water.  Why  does  the  gas  escape  from  soda-water 
as  soon  as  drawn  ?  Why  do  pressure  and  cold  favor  the  solu- 
tion of  a  gas?  Describe  the  diffusion  of  liquids. 

45-7.  Of  gases.  The  osmose  of  liquids.  Of  gases.  Wh\  • 
do  rose-balloons  lose  their  buoyancy  ? 

GRAVITATION. — How  does  Gravitation  differ  from  Cohesion 
and  Adhesion  ?  What  is  the  law  of  gravitation  ?  Why  does 
a  stone  fall  to  the  ground  ?  Will  a  plumb-line  near  a  moun- 
tain hang  perpendicularly?  Why  do  the  bubbles  in  a  cup  of 
tea  gather  on  the  side  ? 

49.  How  is  the  earth  kept  in  its  place?  Define  Gravita- 
tion. Gravity.  Weight.  Give  the  three  laws  of  weight. 

50-2.  What  is  a  vertical  or  plumb-line?  Give  the  four 
laws  of  falling  bodies.  Describe  the  "  guinea  and  feather 
experiment."  What  does  it  prove  ? 

53.  Give  the  equations  of  falling  bodies.  How  can  the  time 
of  a  falling  body  be  used  for  determining  the  depth  of  a  well  ? 
How  does  gravity  act  upon  a  body  thrown  upward  ?  What 
velocity  must  be  given  to  a  ball  to  elevate  it  to  any  point  ? 
How  high  will  it  rise  in  a  given  time  ?  When  it  falls,  with 
what  force  will  it  strike  the  ground  ? 

54-6.  Define  the  Centre  of  Gravity.  The  line  of  direction. 
The  three  states  of  equilibrium.  How  may  the  centre  of 
gravity  be  found  ?  Give  the  general  principles  of  the  centre  of 
gravity.  Describe  the  leaning  tower  of  Pisa. 

57.  Give  some  physiological  applications  of  the  centre  of 
gravity.  Why  do  fat  people  always  walk  so  erect  ? 

58-9.  Define  the  Pendulum.  Arc.  Amplitude.  What  are 
isochronous  vibrations.  Give  the  four  laws  of  the  pendulum. 
Who  discovered  the  first  law  ?  How  ? 

60-2.  What  is  the  centre  of  oscillation?  How  is  it  found? 
Describe  the  pendulum  of  a  clock.  How  is  a  clock  regulated? 
Does  it  gain  or  lose  time  in  winter  ?  Describe  the  gridiron 
pendulum. 

63.  Name  the  various  uses  of  the  pendulum. 

MOTION. — Define  motion,  absolute  and  relative.  Rest, 
Velocity.  Force.  What  are  the  resistances  to  motion  ?  Tell 
what  you  can  about  friction.  Why  does  oil  diminish  friction  ? 

68.  What  uses  has  friction  ?     What  law  governs  the  resist- 


3  2  6  QVESTIONS  iy  PHILOSOPHY- 

ance  of  air  or  water ?  What  is  the  striking  force?  (p.  81, 
Prob.  35.)  What  is  the  tendency  of  gravity?  Define  Mo- 
mentum. 

69.   Show  that  motion  is  not  imparted  instantaneously. 

70-1.  Give  the  three  laws  of  motion  and  the  proof  of  each. 
If  a  ball  be  fired  into  the  air  when  a  horizontal  wind  is  blow- 
ing, will  it  rise  as  high  as  if  the  air  were  still  ?  Define  com- 
pound motion. 

72.  Define  the  "parallelogram  of  forces."     The  resultant. 
How  can  the  resultant  of  two  or  more  forces  be  found  ?     Give 
practical  illustrations  of  compound  motion. 

73.  WThat  is  the  "resolution  of  forces?"     Show  how  one 
vessel  can  sail  south  and  another  north,  driven  by  the  same 
westerly  wind. 

74-5.  Explain  how  a  kite  is  raised.  Explain  the  towing  of 
a  canal-boat.  Define  circular  motion. 

76.  Apply  the  principle  of  circular  motion  to  the  revolu- 
tion of  the  earth  about  the  sun. 

77.  Show  when  the  centrifugal  force  becomes  strong  enough 
to  overcome  the  force  of  Cohesion,  Adhesion,  Gravity.     What 
effect  does  the  revolution  of  the  earth  on  its  axis  have  upon 
all  bodies  on  the  surface  ?     W'hat  would  be  the  effect  if  the 
rotation  were  to  cease  ?     Describe  the  action  of  the  centrifugal 
force  on  a  hoop  rapidly  revolved  on  its  axis. 

78.  Give  practical  illustrations  of  action  and  reaction.     If  a 
bird  could  live,  could  it  fly  in  a  vacuum  ? 

79.  Define  reflected  motion.     Give  its  law. 

80.  How  is  curved  motion  produced  ?     Is  perpetual  motion 
practicable  ? 

THE  MECHANICAL  POWERS. — Name  and  define  the  ele- 
ments of  machinery.  Do  the  "  powers,"  so  called,  produce 
force  ?  What  is  the  law  of  Mechanics  ?  Illustrate  the  law. 

86-7.  Describe  the  three  classes  of  levers.  The  law  of 
equilibrium. 

88.  What  is  the  advantage  peculiar  to  each  class  ?    Describe 
the  steelyard  as  a  lever.     What  effect  does  it  have  to  reverse 
the  steelyard  ? 

89.  Describe  the  arm  as  a  lever.      Would  a  lever  of  the 
first  class  answer  the  purpose  of  the  arm  ?      What  is  a  bent 
lever  ? 

90.  Describe  the  compound  lever.     The  wheel  and  axle. 

91.  The  capstan.      Give  the  law  of  equilibrium.     What  is 
the  advantage  of  the  wheel  and  axle  ? 


QUESTIONS  IN  PHILOSOPHY.  327 

92.  Describe  a  system  of  wheel-work.  At  which  arm  of 
the  lever  is  the  P.  applied  ? 

93-4.  Describe  the  various  uses  of  the  inclined  plane.  Its 
law  of  equilibrium.  What  velocity  does  a  body  acquire  in 
rolling  down  an  inclined  plane?  Give  illustrations. 

95.  Describe  the  screw.     Its  uses.     Its  law  of  equilibrium 
How  may  its  power  be  increased  ?     What  limit  is  there  ? 

96.  Describe  the  wedge.     Its  uses.     Its  law  of  equilibrium. 
How  does  it  differ  from  that  of  the  other  powers  ? 

97.  Describe   the   pulley.      The  use  of  fixed   pulleys.      Is 
there  any  gain  of  P.  in  a  fixed  pulley? 

98.  The  use  of  a  movable  pulley.     Describe  a  movable  pul- 
ley as  a  lever. 

99.  Give  the  general  law  of  equilibrium  in  a  combination  of 
pulleys.     What  part  of  the  force  is  lost  by  friction  ? 

HYDROSTATICS.— Define.  What  liquid  is  taken  as  the 
type  ?  What  is  the  first  law  of  liquids  ?  Explain.  Illustrate 
the  transmission  of  pressure  by  water. 

105.  Show  how  water  is  used  as  a  mechanical  power. 

106.  Describe  the  hydrostatic  press.     Give  its  law  of  equi- 
librium. 

107.  What  are  the  uses  of  this  press?     What  pressure  is 
sustained  by  the  lower  part  of  a  vessel  of  water,  when  acted  on 
by  gravity  alone  ?     How  does  this  pressure  act  ? 

108.  Give  the  four  laws  which  depend  on  this  principle,  and 
illustrate  them.     What  is  the  weight  of  a  cubic  foot  of  sea- 
water  ?     Fresh  water  ?     What  is  the  pressure  at  two  feet  ? 

109.  Give  illustrations  of  the  pressure  at  great  depths. 

1 10.  Describe   the   hydrostatic   bellows.     Its   law  of  equi- 
librium. 

111.  What  is  the  hydrostatic  paradox ?     Give  illustrations. 
Give  the  principle  of  fountains.     How  high  will  the  water  rise? 

112-3.  How  do  modern  engineers  carry  water  across  a 
river?  Did  the  ancients  understand  this  principle?  Give  the 
theory  of  the  Artesian  well. 

114.  Give  the  rule  for  finding  the  pressure  on  the  bottom 
of  a  vessel.     On  the  side. 

115.  Define  the  water-level.     Is  the  surface  of  water  hori- 
zontal?    If  it  were,  what  part  of  an  approaching  ship  would 
we  see  first  ?     Describe  the  spirit-level.     Define  specific  gravi- 
ty.    What  is  the  standard  for  solids  and  liquids?     For  g^ses  i 

116-7.    Explain   the   buoyant  force  of  liquids.      What  is 


2  2 8  QUESTIONS  IK  PHILOSOPHY. 

Archimedos's  law?     Describe  the   "cylinder  and  bucket  ex- 
periment."    What  does  it  prove  ? 

118.  Give  the  method  of  finding  the  specific  gravity  of  a 
solid.     A  liquid.       Suppose  the    solid  is  lighter  than  water 
and  will   not   sink,  what   can   you   do?     Ans.    Tie   a  heavy 
solid  to  it,  and  then  make  allowance  for  this  in  calculating  the 
specific  gravity.     Explain  the  hydrometer. 

1 19.  How  can  you  find  the  weight  of  a  given  bulk  of  any 
substance  ?     The  bulk  of  any  given  weight  ?     The  exact  vol- 
ume of  a  body  ? 

1 20- 1.  Illustrate  the  action  of  dense  liquids  on  floating 
bodies.  Why  will  an  iron  ship  iloat  on  water?  Where  is 
the  centre  of  gravity  in  a  floating  body?  How  do  fish  sink  at 
pleasure  ? 

HYDRAULICS — Define.  To  what  is  the  velocity  of  a  jet 
equal?  How  is  the  velocity  found?  Give  the  rule  for  finding 
the  quantity  of  water  which  can  be  discharged  from  a  jet  in  a 
given  time. 

124.  What  is  the  effect  of  tubes  ?  Tell  something  of  the 
flow  of  water  in  rivers. 

125-7.  Name  and  describe  the  different  kinds  of  water- 
wheels.  Which  is  the  most  valuable  form?  What  is  the 
principle  of  the  Turbine?  Describe  Barker's  Mill. 

128-9.  How  are  waves  produced?  Explain  the  real  motion 
of  the  wave.  How  does  the  motion  of  the  whole  wave  differ 
from  that  of  each  particle  ?  How  is  the  character  of  waves 
modified  near  the  shore  ? 

130.  What  is  the  extreme  height  of*  "mountain  waves?" 
Define  like  phases.  Unlike  phases.  A  wave-length.  What 
is  the  effect  if  two  waves  with  like  phases  coincide  ?  With 
unlike  phases?  What  is  this  termed? 

PNEUMATICS. — Define.  What  principles  are  common  to 
liquids  and  gases?  What  gas  is  taken  as  the  type?  De- 
scribe the  air-pump.  Can  a  perfect  vacuum  be  obtained  in 
thi  /way  ?  Prove  that  the  air  has  weight. 

134-5.  Show  its  elasticity  and  compressibility.  Describe 
the  bottle-imps.  What  principles  do  they  illustrate?  Show 
the  expansibility  of  the  air. 

136.  Describe  the  experiments  with  the  hand-glass.     The 
Magdeburg  hemispheres.     What  do  they  prove  ? 

137.  Show  the  upward  pressure  of  the  air.     The  buoyant 
force  of  the  air.     Would  a  pound  of  feathers  and  a  pound  o< 


QUESTIONS  IN  PHIL OSOPHY.  3  2  9 

lead  balance,  if  placed  in  a  vacuum  ?     On  what  principle  does 
a  balloon  rise  ? 

138-140.  What  is  the  amount  of  the  pressure  of  the  air? 
Describe  the  experiment  illustrating  this.  Where  do  these 
figures  apply?  Describe  how  the  pressure  of  the  air  con- 
stantly varies. 

141.  Give  Mariotte's  law.      Describe  the  barometer.      Its 
uses.     Are  the  terms  "fair,"  *«  foul,"  etc.,  often  placed  on  the 
scale,  to  be  relied  upon  ? 

142.  Why  is  mercury  used  for  filling  the  barometer?     De- 
scribe Otto  Guericke's  barometer. 

143-4.  Describe  the  action  of  the  lifting-pump.  The  force- 
pump.  The  fire-engine. 

145-6.  The  siphon.     Explain  its  theory. 

147.  The  pneumatic  inkstand.    What  was  the  view  of  the 
ancients  concerning  the  pressure  of  the  air  ?     Tell  the  story  of 
Galileo.     What  opposing  forces  act  on  the  air? 

148.  How  high  does  the  air  extend?     How  does  its  density 
vary  ? 

ACOUSTICS. — Define.  Name  and  define  the  two  senses  ol 
this  word.  May  not  "light,"  "heat,"  etc.,  be  used  in  the 
same  way  ?  Illustrate  the  formation  of  sound  by  vibrations. 

152.  Show  how  the  sound  of  a  tuning-fork  is  conveyed 
through  the  air.  Report  of  a  gun.  The  sound  of  a  bell. 
The  human  voice.  Define  a  sound-wave.  A  wave-length. 
In  which  direction  do  the  molecules  of  air  vibrate?  In  what 
form  do  the  waves  spread  ?  Can  a  sound  be  made  in  a  vacuum  ? 

155.  Can  a  sound  come  to  the  earth  from  the  stars  ?     How 
do  sounds  change  as  we  pass  above  or  below  the  sea-level  ? 
Upon  what  does  the  velocity  of  sound  depend  ?     Why  is  this? 

156.  At  what  rate  does  sound  travel  in  the  air?     In  water? 
In  iron  ?     What  effect  does  temperature  have  on  the  velocity 
of  sound  ?     Describe  Biot's  experiment  in  the  water-pipes  of 
Paris.     Do  all  sounds  travel  at  the  same  rate  ? 

157-8.  How  does  the  velocity  of  sound  enable  us  to  deter- 
mine distance  ?  Upon  what  does  the  intensity  of  sound  de- 
pend? At  what  rate  does  it  diminish ?  Why?  State  where- 
in the  laws  of  sound  are  similar  to  those  of  other  phenomena. 
What  does  this  uniformity  prove?  Explain  the  speaking-tube. 

159.  The  ear-trumpet.     The  speaking-trumpet.     \Vhat  is 
the  refraction  of  sound  ? 

160.  Define  reflection  of  sound.     What  is  the  law  ?     Give 


330 


VUE3T10NS  IN  rUILOSOPUY. 


curious  instances   of  reflection.      What   is   the   shape   of  a 
whispering-gallery  ? 

161.  Illustrate  the  decrease  of  sound  by  repeated  reflection. 
Why  are  sounds  more  distinct  at  night  than  by  day  ?     What 
is  a  resonance  ? 

162.  Is  it  desirable  to  have  a  door  or  a  window  behind  a 
speaker?     What  causes  the  "ringing"  of  a  sea-shell?     When 
is  an  echo  heard?     When  is  the  echo  repeated? 

163.  What   is   the  difference   between   noise   and   music? 
Upon  what  does  pitch  depend? 

164-6.  Describe  the  siren.  How  is  it  used  to  determine  the 
number  of  vibrations  in  any  sound?  How  is  the  octave  of 
any  note  produced  ?  How  can  we  ascertain  the  length  of  the 
wave  in  sound  ? 

167.  What  length  of  wave  produces  the  low  tones  in  music? 
The   high   tones?     Give   the   illustration   of  the   locomotive 
whistle.     When  are  two  tones  in  unison  ?     How  can  we  find 
the  length  of  the  wave  in  any  musical  sound? 

1 68.  What  is  the  length  of  the  wave  in  a  man's  voice  in 
common  conversation  ?     How  can  two  sounds  produce  silence  ? 
What  is  this  effect  termed? 

169.  Illustrate  interference  by  means  of  a  tuning-fork.     De- 
scribe the  vibration  of  a  cord. 

170-1.  Describe  the  sonometer.  What  is  the  object  of  the 
wooden  box  ?  Give  the  three  laws  of  the  vibration  of  cords. 
What  is  a  node  ? 

172.  Describe  the  experiments  illustrating  the  formation  of 
nodes. 

173.  What  are  acoustic  figures ?     Nodal  lines? 

175.  What   is   the  fundamental   tone  of  a  cord?     A  har- 
monic ?     What  causes  the  difference  in  the  sound  of  various 
instruments?     Does  a  bell  vibrate   in  nodes?      The  violin- 
case  ?     A  piano  sounding-board  ? 

176.  Give  the  fractions  representing  the  relative  rates  of 
vibration  of  the  different  notes  of  the  scale.     How  is  the  sound 
produced  in  wind-instruments  ? 

177.  How  is  the  sound-wave  started  in  an  organ-pipe?     In 
a  flute  ?     What  determines  the  pitch  ? 

178.  What  are  sympathetic  vibrations  ?     Describe  the  ear. 

1 79.  What  is  the  object  of  the  Eustachian  tube  ?     Is  there 
any  opening  between  the  external  and  internal  ear?     What 
effect  does  it  have  on  the  hearing  to  increase  or  diminish  the 


QUESTIONS  IN  PHILOSOPHY.  3  j  } 


pressure  of  the  ail  ?     How  does  a  concussion  sometimes 
temporary  deafness  ?     How  can  this  be  remedied  ? 

1  80.  What  are  the  limits  of  hearing?  Does  the  range  vary 
in  different  persons  ?  What  sounds  are  generally  most  acutely 
heard  ? 

181.  Are  there  probably  sounds  in  nature  we  never  hear? 
Has  nature  a  tendency  to  music?     What  causes  the  "  whisper- 
ing of  the  pines  ?" 

182.  What  is  the  key  of  nature?     What  are  sensitive  flames? 
How  can  a  flame  be  made  to  sing?     What  causes  the  song? 

OPTICS.  —  Define.  A  luminous  body.  A  non-luminous 
body.  A  medium.  A  transparent  body.  A  translucent 
body.  An  opaque  body.  A  ray  of  light.  Show  that  neither 
air  nor  water  is  perfectly  transparent.  Why  is  the  sun's  light 
fainter  at  sunset  than  at  mid-day  ? 

1  88.  Define  the  visual  angle.  Show  how  distance  and  si:e 
are  intimately  related.  Give  the  laws  of  light.  Do  they  re- 
semble those  of  sound? 

189.  What  is  the  velocity  of  light?     How  is  this  proved? 
Explain  the  undulatory  theory  of  light. 

190.  How    does    light-motion    differ   from    sound-motion  ? 
What  is  diffused  light  ?     Why  are  some  objects  brilliant  and 
others  dull  ?     Why  can  we  see  a  rough  surface  at  any  angle, 
and  an  image  in  the  mirror  at  only  a  particular  one  ?     Would 
a  perfectly  smooth  mirror  be  visible?      How  does  reflection 
vary  ?     Define  mirrors.     Name  and  define  the  three  kinds. 
What  is  the  action  of  each  on  rays  of  light  ?     What  is  the 
general  principle  of  mirrors  ? 

192-3.  Why  is  an  image  in  a  plane  minor  symmetrical? 
Why  is  it  reversed  right  and  left?  Why  is  it  as  far  behind 
the  mirror  as  the  object  is  before  it  ?  If  you  sit  where  you 
cannot  see  another  person's  image,  why  cannot  that  person  s^e 
yours  ?  Why  can  we  often  see  in  a  mirror  several  images  of 
an  object  ?  Why  can  we  see  these  best  if  we  look  into  the  mir- 
ror very  obliquely?  Why  is  an  image  seen  in  water  inverted  ? 

194.  When  the  moon  is  near  the  meridian  why  can  we  see 
the  image  in  the  water  at  only  one  spot  ?     When  do  we  see  a 
tremulous  line  of  light?     Define  the  focus.     Centre  of  curva- 
ture. 

195.  Describe  the  image  seen  in  a  concave  mirror.     Why 
is  it  inverted  when  we  stand  between  the  centre  of  curvature 
and  the  principal  focus  ?     Why  is  it  larger  than  life  when  we 
stand  within  the  principal  focus,  and  smaller  than  life  when 
we  stand  without  the  centre  of  curvature  ? 


33* 


QUESTIONS  IN  PHILOSOPHY. 


196.  What  are  conjugate  foci  ?     Describe  the  image  seen  in 
a  convex  mirror.     Why  is  it  smaller  than  life  ?    Why  can  it 
not  be  inverted  like  one  seen  in  a  concave  mirror?    Ans.  Be- 
cause the  rays  do  not  cross  each  other. 

197.  Define  total  reflection.     Define  Refraction.     Does  the 
partial  reflection  of  light  as  it  passes  from  one  medium  to 
another  of  different  density  have  a  parallel  in  sound  ?    Why  is 
powdered  ice  opaque  while  a  block  of  ice   is   transparent? 
Give  illustrations  of  refraction. 

198.  Why  does  an  object  in  water  appear  to  be  above  it> 
true  place  ?     What  is  the  general  principle  of  refraction  ? 

199.  Give  the  laws  of  refraction.     Describe  the  path  of  «t 
ray  through   a  window-glass.      Is   the    direction  of    objects 
changed?     Describe  the  path  through  a  prism. 

200.  Name  and  describe  the  different  kinds  of  lenses.    What 
is  the  effect  of  a  double  convex  lens  on  rays  of  light  ? 

201.  What  is  this  kind  of  lens  often  called?     Describe  the 
image.     Why  is  it  inverted  after  we  pass  the  principal  focus  ? 
Why  is  it  decreased  in  size  ? 

202.  What  is  the  effect  of  a  double  concave  lens  on  rays  ot 
light?     Describe  the  image.     Why  can  it  not  be  inverted  like 
one  through  a  double  convex  lens  ?     Describe  the  images  seen 
in  the  large  vases  in  the  windows  of  drug-stores.     What  is  a 
mirage  ? 

203.  Give  its  cause. 

204.  How  is  the  solar  spectrum  formed  ?     Name  the  seven 
primary  colors.     Show  that  these  seven  will  form  white  light. 
What  other  opinions  are  held  ? 

205.  Why  are  the  rays  separated?     What  is  meant  by  the 
dispersive  power  of  a  prism  ?     What  substance  possesses  this 
property  in  the  highest  degree  ? 

206.  What   three  classes  of  rays  compose  the  spectrum? 
Do  artificial  lights  differ  in  their  proportion  of  these  rays? 
What  color,  also,  predominates?     Ans.   Yellow.     Why  does 
the  window  of  a  photographer's  dark  room  sometimes  contain 
yellow  glass  ?     Define  complementary  colors.     How  can  they 
be  seen?    What  is  the  effect  of  complementary  colors  when 
brought  in  contrast?     (In  Fig.  153,  opposite  colors  are  com- 
plementary. )     Ought  a  red  flower  to  be  placed  in  a  bouquet 
by  an  orange  one  ?     A  pink  or  blue  with  a  violet  one  ?    Why 
do  colors  seen  by  artificial  light  appear  differently  than  by 
daylight — as  yellow  seems  white,  blue  turns  to  green,  etc.  ? 

207-8.  Describe  Newton's  rings.     How  are  these  explained 


qUESTIONS  IN  PHILOSOPHY.  333 

according  to  the  wave-theory?     What  can  you  say  about  the 
length  of  the  waves  ? 

209.  State  the  analogy  betwten  color  and  pitch  in  music. 
Why  is  grass  green?    When  is  a  body  white?,  Black?    What 
causes  the  play  of  color  in  mother-of-pearl  ?    In  soap-bubbles  ? 
In  the  scum  on  stagnant  water  ?     In  thin  layers  of  mica  or 
quartz  ?     What  is  a  tint  ? 

210.  Define  diffraction.    What  is  double  refraction?    What 
are  the  two  rays  termed  ?     What  is  polarized  light  ? 

211.  How  does  a  dot  appear  through  Iceland  spar?     What 
other  methods  of  polarizing  light?     Give  some  illustrations 
and  practical  uses  of  polarized  light. 

212-3.  How  is  the  rainbow  formed  ?  Why  must  it  rain  and 
the  sun  shine  at  the  same  time,  to  produce  the  bow  ?  Why  is 
the  bow  in  the  sky  opposite  the  sun  ?  How  many  refractions 
and  reflections  forrn  the  primary  bow  ?  The  secondary  ?  How 
many  colors  can  one  receive  from  a  single  drop  ?  Why  is  the 
bow  circular  ? 

214.  How  are  halos  formed?  What  is  the  cause  of  the 
" sun's  drawing  water?"  Explain  spherical  aberration. 

.215.  Chromatic  aberration.  Its  remedy.  What  is  the 
meaning  of  the  word  microscope  ?  Describe  the  simple  mi- 
croscope. The  compound  microscope.  How  is  the  power  of 
a  microscope  indicated  ?  Do  we  see  the  object  directly  in  a 
microscope  ?  Why  is  the  object-lens  made  so  small  and  so 
convex  ? 

216-8.  What  is  the  meaning  of  the  word  telescope?  De- 
scribe the  reflecting  telescope.  The  refracting  telescope. 
What  is  the  use  of  the  object-lens  ?  The  eye-piece  ?  Is  the 
image  inverted  ?  Describe  the  opera-glass. 

219.  The  stereoscope.     The  magic  lantern.     How  are  dis- 
solving views  produced  ? 

220.  Describe  the  Camera.     The  structure  of  the  eye. 

222.  The  formation  of  an  image  on  the  retina.     The  ad- 
justment of  the  eye.     The  cause  of  near  and  far  sightedness. 
The  remedy.  Why  do  old  people  hold  a  book  at  arm's-length  ? 

223.  Illustrate  the  duration  of  an  impression.     Why  are  we 
not  sensible  of  darkness  when  we  wink  ?     Why  can  we  not  see 
the  fence-posts  when  we  are  riding  rapidly?     Describe  color- 
blindness.    Whit  is  the  range  of  the  eye? 

HEAT. — Define  luminous  heat.  Obscure  heat.  A  dia- 
thermanous  body.  Cold.  Gases  and  vapors.  Show  the  in- 
timate relation  between  light  and  heat. 


3  3  4  <?  UESTIONS  IN  PJ1IL  OSOPHT. 


228-9.  What  is  light  ?  How  do  the  three  classes  of  rays 
in  the  solar  spectrum  differ  ?  What  effect  does  each  of  these 
produce?  What  is  the  theory  of  heat  ?  Why  can  we  not  see 
with  our  fingers  or  taste  with  our  ears  ?  At  what  rate  does 
nerve-motion  travel  ?  How  long  does  it  take  a  tall  man  to 
find  out  what  is  going  on  in  his  foot  ?  What  is  meant  by  the 
quality  of  heat  ?  Does  this  find  any  analogy  in  sound  ? 

230.  Name  the  sources  of  heat.  Describe  and  illustrate 
each  of  these.  Can  force  be  destroyed  ?  If  apparently  lost, 
what  becomes  of  it  ?  What  is  Joule's  law? 

232.  Define  latent,  sensible,  and  specific  heat. 

233.  Explain  the  paradox,  "  that  freezing  is  a  warming  pro- 
cess and  thawing  a  cooling  one."      Explain   the  action  of  a 
freezing  mixture.     Why  does  heat  expand  and  cold  contract  ? 
What  do  you  say  as  to  the  uniformity  of  the  expansion  of 
solids,  liquids,  and  gases  ? 

234.  Illustrate  the  expansion  of  solids.     Is  it  better  to  buy 
alcohol  in  summer  or  in  winter  ? 

235.  What  is  the  thermometer?      Describe  it.      Describe 
the  process  of  filling  and  grading. 

236.  The  F.,  C,  and  R.  scales.     Tell  what  you  can  about 
liquefaction.     Of  a  solid.     Of  a  gas.      In   one   case   sensible 
heat  becomes  latent,  in  the  other  latent  heat  becomes  sensi- 
ble —  why  is  this  ? 

237.  Give  the  theory  of  vaporization.     Distillation.     Since 
rain  comes  from  the  ocean,  why  is  it  not  salt  ? 

238-9.  Describe  the  theory  of  boiling.  What  is  the  boil- 
ing point  ?  Do  all  liquids  boil  at  the  same  temperature  ? 
What  would  be  the  effect,  if  this  were  the  case  ?  Upon  what 
does  the  boiling-point  depend  ?  Why  does  salt-water  boil  at 
a  higher  temperature  than  fresh-water  ?  Why  will  milk  boil 
over  so  easily  ?  Why  will  soup  keep  hot  longer  than  boiling 
water  ?  Does  the  air,  dissolved  in  water,  have  any  influence 
on  the  boiling-point?  (Page  247.)  Can  you  measure  the 
height  of  a  mountain  by  means  of  a  tea-kettle  and  a  ther- 
mometer ?  Show  how  cold  water  may  be  used  to  make  warm 
water  boil. 

240.  At  what  temperature  will  water  boil  in  a  vacuum? 
Why?     Describe  the  water-hammer  and  the  pulse-glass. 

241.  Can  we  heat  water  in  the  open  air  above  the  boiling- 
point  ?     What  becomes  of  the  extra  heat  ?     What  is  the  latent 
heat  of  water  ?     Upon  what  principle  are  buildings  heated  by 
steam  ?     Have  you  ever  seen  any  steam  ?     Define  evaporation. 
Does  snow  evaporate  in  the  winter  ?     What  can  be  done  to 


q  UES  T10NS  IN  PHIL  OSOPHY.  3  3  5 

hasten  evaporation  ?  Why  is  a  saucepan  made  broad  ?  Why 
do  we  cool  ourselves  by  fanning  ?  Why  does  an  application 
of  spirits  to  the  forehead  allay  fever  ?  Why  does  wind  hasten 
the  drying  of  clothes  ?  Describe  a  vacuum-pan.  Why  is 
evaporation  hastened  in  a  vacuum  ? 

242.  Why  is  evaporation  a  cooling  process  ?      How  is  ice 
manufactured  in  the  tropics?     What  is  the  spheroidal  state? 

243.  Name  and  define  the  three  modes  of  communicating 
heat.      Give   illustrations   showing    the    relative   conducting 
power  of  solids,  liquids,  and  gases.     What  substances  are  the 
best  conductors? 

244.  Is  water  a  good  conductor  ?     Air  ?     What  is  the  prin- 
ciple of  ice-houses  ?     Fire-proof  safes  ?     Why  do  not  flannel 
and  marble  appear  to  be  of  the  same  temperature  ?     Is  ice 
always  of  the  same  temperature,  or  is  some  ice  colder  than 
others  ?     Describe  the  convective  currents  in  heating  water. 
Where  must  the  heat  be  applied  ?     Where  should  ice  be  ap- 
plied in  order  to  cool  water  ? 

245.  Describe  the  convective  currents  in  heating  air.     Upon 
•what  principle  are  hot-air  furnaces  constructed?     Ought  the 
ventilator  at  the  top  of  a  room   to   be   opened   in   winter? 
At  the  bottom  ?     Is  space  warmed  by  the  sunbeam  ?     Show 
how  the  glass  in   a   hothouse   acts   as   a  trap  to  catch   the 
sunbeam.     Does  the  heat  of  the  sun  come  in  through  our 
windows  ?     Does  the  heat  of  our  stoves  pass  out  in  the  same 
way?     Ans.    It  does,  but   only  through   absorption  by  the 
glass,  and  not  by  direct  radiation  from  the  fire.     Show  how 
the  vapor  in  the  air  helps  to  keep  the  earth  warm.      The 
top  of  a  mountain  is  nearer  the  sun,  why  is  it  not  warmer? 
(Page  249. )     Why  does  ice  form  at  night  on  the  Desert  of 
Sahara  ?     Explain  the  relation  between  absorption  and  reflec- 
tion.    Is  a  dusty  boot  hotter  to  the  foot  than  a  polished  one  ? 
What  is  the  elastic  force  of  steam  at  the  ordinary  pressure  of 
the  air  ?   What  is  the  difference  between  a  high-pressure  and  a 
low-pressure  engine?   Which  is  used  for  a  locomotive?   Why? 

247.  Describe  the  governor.     What  is  the  object  of  a  fly- 
wheel ? 

248.  How  does  the  capacity  of  the  air  for  moisture  vary  ? 
What   is   the   principle   on   which    dew,  rain,  etc.,  depend? 
Show  that  a  change  in  density  produces  a  change  in  tempera- 
ture.    What  effect  does  this  have  on  the  temperature  of  eleva- 
ted regions  ? 

250.  How  is  dew  formed  ?     Upon  what  objects  will  it  collect 


j^6  QUESTIONS  IN  PHILOSOPHY. 

most  readily?  Why  will  it  not  form  on  windy  nights?  Is  a 
heavy  dew  a  sign  of  rain?  Ans.  Yes,  because  it  shows  that 
the  moisture  of  the  air  is  easily  condensed.  The  "sweating 
of  a  pitcher?"  What  is  frost?  Ans.  Frozen  dew.  Wh> 
will  a  slight  covering  protect  plants  from  frost?  Ans.  Be- 
cause it  prevents  radiation.  Why  is  there  no  frost  on  cloudy 
nights?  Ans.  The  clouds  act  like  a  blanket,  to  prevent  radia- 
tion and  keep  the  earth  warm.  What  is  a  fog  ? 

251.  How  does  a  fog  differ  from  a  cloud  ?     Why  are  moun- 
tains "  cloud-capped?"     Why  do  clouds  remain  suspended  in 
the  air,  contrary  to  gravity  ? 

252.  Describe  the  different  kinds  of  clouds.     Describe  the 
formation  of  rain. 

253-4.  Snow.  Winds.  Land  and  sea  breezes.  Trade- 
winds.  Oceanic  currents.  Tell  about  the  Gulf  Stream.  Ex- 
plain the  influence  which  water  has  on  climate.  Of  what 
practical  use  is  the  air  in  water  ? 

256.  Describe  the  exception  which  exists  in  the  freezing  of 
water.  Why  is  this  made  ?  Describe  the  two  processes  by 
which  pure  water  can  be  obtained.  How  is  an  excessive  de- 
posit of  dew  prevented  ? 

ELECTRICITY. — Give  the  origin  of  this  word.  Name  the 
different  kinds  of  Electricity.  Define  Magnetism.  A  Magnet. 
A  natural  magnet.  An  artificial  one.  A  bar-magnet. 

262.  A  horse-shoe  magnet.  The  poles.  The  magnetic  curves. 

264.  Describe  a  magnetic  needle.     What  is  the  law  of  mag- 
netic attraction  and  repulsion  ?     Define  magnetic   induction. 
Explain  it.     When  is  a  body  polarized  ?     Give  some  illustra- 
tions of  induced  magnetism. 

265.  Does  a  magnet  lose  any  force  by  induction?     How  do 
you  explain  the  fact  that  if  you  break  a  magnet  each  part  will 
have  its  N.  and  S.  pole  ?     Describe  the  process  of  making  a 
magnet.     On  what  principle  will  you  explain  this  ? 

266.  Describe  the  compass.      Is  the  needle  true  to  the 
pole  ?     What  causes  it  to  vary  ?     What  is  the  line  of  no  varia- 
tion ?     Declination  ? 

267.  Why  does  the  needle  point  N.  and  S.  ?     What  is  a 
dipping-needle  ?     Explain.     How  is  a  needle  balanced  ? 

268.  Where  is  the  N.  magnetic  pole  ?    How  could  one  know 
when  he  reaches  it?      Does  the  earth    induce    magnetism? 
Which  end  of  an  upright  bar  will  be  the  S.  pole  ?     How  has 
the  lodestone  become  polarized? 

269.  Define  factional  electricity.     The  electroscope.     Dif- 


Q UEST10NS  IN  PHIL OSOPHY.  ^  3  7 

ference  between  static  and  dynamic  electricity.  Show  the 
existence  of  two  kinds  of  electricity.  Give  the  names  applied 
to  each. 

271.  State  the  law.  What  is  the  theory  of  electricity?  Is 
it  a  polar  force  ?  Is  it  easily  disturbed  ?  Define  a  conductor. 
An  insulator. 

272-3.  What  is  the  best  conductor  ?     Best  insulator  ?     Is  a 
poor  conductor  a  good  insulator  ?     When  is  a  body  said  to  be- 
insulated?     Can  electricity  be  collected  from  an  iron  rod? 
Describe  an  electrical-machine.     What  is  the  use  of  the  chain  , 
in  the  negative  pole  ? 

274.  Define  electrical  induction. 

275.  Faraday's  theory. 

276.  Describe  the  electric  chime.     Explain. 

277.  The  dancing  images.     The  Leyden  jar.     What  gives 
the  color  to  the  spark  ? 

278.  How  is  the  jar  discharged  ?  What  are  the  essentials  of  a 
Leyden  jar  ?     What  is  the  object  of  the  glass  ?     The  tin-foil  ? 

279.  Give  the  theory  of  the  charging  of  the  jar.     Can  an 
insulated  jar  be  charged  ?     Is  the  electricity  on  the  surface  or 
in  the  glass  ?     Can  the  inner  molecules  of  a  solid  conductor 
be  charged  ?     Will  a  rod  contain  any  more  electricity  than  a 
tube  ?     Why  is  the  prime  conductor  of  an  electrical-machine 
hollow  ?    What  is  the  effect  of  points  ?     How  can  we  test  this  ? 

280.  Describe  the  electric  whirl.     Explain  the  existence  of 
electricity  in  the  atmosphere. 

281.  What  is  the  cause  of  lightning?     Thunder?     Is  there 
any  danger  when  you  once  hear  the  report?     Describe  the 
different  kinds  of  lightning.     Tell  how  Franklin  discovered 
the  identity  of  lightning  and  frictional  electricity. 

282.  What  is  the  cause  of  the  Aurora  Borealis?     How  is 
this  shown  ?     Prove  the  intimate  relation  between  the  aurora 
and  magnetism.     What  are  Geissler's  tubes?     Gassiot's  cas- 
cade? 

283.  Tell  what  you  can  about  lightning-rods. 

284.  In  what  consists  the  main  value  of  the  rod?     Does 
the  lightning  ever  pass  upward  from  the  earth?      Ans.  It 
does,  both  quietly  and  by  sudden  discharge.     Has  Nature  pro- 
vided any  lightning-rods  ?     What  is  St.  Elmo's  fire  ?     What 
is  the  velocity  of  electricity? 

285-6.  Illustrate  its  instantaneousness.  Name  some  of  the 
effects  of  frictional  electricity — (i)  Physical,  (2)  Chemical, 
(3)  Physiological. 

287.  How  are  galvanic  electricity  and  chemistry  related? 


3  -  S  qiTESTIONS  IN  PHILOSOPHY. 

Why  is  galvanic  or  voltaic  electricity  thus  named  ?     Tell  the 
story  of  Galvani's  discovery.     What  was  his  theory? 

288.  Give  an  account  of  Volta's  discovery*     What  was  his 
theory?     How  can  we   form   a   simple   pile?      Describe   the 
simple  galvanic  circuit. 

289.  Define  the  poles.     Electrodes.     Closing  and  breaking 
the  circuit.     What  is  necessary  to  form  a  voltaic  pair? 

290.  Are  the  terms  applied  to  the  metals  the  same  as  those 
to  the  poles  ?      Give  the  chemical  change.      Why  doos  the 
hydrogen  come  off  from  the  copper? 

291.  Tell  what  you  can  about  the  current.      What  really 
passes  along  the  wire  ?     How  is  this  force  transmitted  ?     Will 
a  tube,  then,  convey  as  much  electricity  as  a  rod  ? 

292.  Describe  Smee's    battery.      Grove's    battery.      The 
chemical  change. 

293.  The  advantages  of  Grove's  battery.     Describe  Bun- 
sen's  battery.     Daniell's  battery. 

294.  Sulphate  of  copper  battery.     Define  quantity  and  in- 
tensity.    Upon  what  do  they  depend?      Compare  frictional 
and  galvanic  electricity. 

295.  Give  the  effects  of  galvanic  electricity,  (i)  Physical — 
heat  and  light ;  (2)  Chemical — decomposition  of  water,  elec- 
trolosis,  electrotyping,  duplicates  of  copper-plates,  and  elec- 
tro-plating;  (3)  Physiological. 

302-3.  What  is  the  effect  of  a  voltaic  current  on  a  magnetic 
needle  ?  What  is  a  galvanometer  ? 

304.  An  astatic  needle  ?     An  electro-magnet  ?     A  helix  ? 

305.  Show  how  a  helix  can  be  magnetized.     How  are  bar- 
magnets   made  ?      How  is   motion   produced  by  electricity  ? 
Describe  Page's  rotating-machine. 

306.  What  is  the  principle  of   an  electric  engine  ?     What 
difficulty  is  there  in  the  way  of  its  practical  use  ?     Describe 
the  magnetic  telegraph. 

307.  How  is  a  message  sent  ?     How  is  one  received  ? 
308-9.  What  is  a  sounder  ?  What  is  the  general  principle  of 

the  telegraph?     Describe  the  relay.     Name  the  use  of  each 
instrument. 

310-11.  Define  magnetic  electricity.  Describe  a  magneto- 
electric  machine.  Describe  Wilde's  machine.  Induced  cur 
rents. 

312.  Ruhmkorft's  coil. 

313.  Thermal  electricity.     A  thermo-electric  pile. 

314.  Describe  the  electric  fish. 


INDEX. 


"          figures  
Action  and  Reaction... 
Adhesion  
Air        

173 
78 
40 
132 
133 
118 
58 
121) 
39 
81) 
112 
132 
11) 
33 
40 
36 
11 
48 
282 

127 
141 

.".»:! 

2U2 
2<J3 

313 
175 

244 
32 

220 
41 
91 
134 
54 
60 
35 
215 
,  <>4 
25  L 

88 

312 
'>!W 

Diathermancy  

227 
210 
44 
45 
237 
17 
27 

178 
159 
161 

29 
292 
296 
306 
280 
273 
259 
269 
287 
261 
289 
300 
302 
304 

298 
3UO 
298 
29S 
269 
54 
241 
233 
220 

50 
222 
23 
145 
182 
182 
119 
200 
250 
67 
144 
75 
75 
72 
35 
73 
112 
233 
256 
67 

303 
227 
44 
137 
«,SO 

"        Elasticity  of.  
"       Osmose  of.  
"        Pressure  of  

2# 
46 
132 
283 
283 

$ 

48 
49 
54 
115 
254 

214 
31 
175 
225 

24S 
246 
243 
245 
233 
232 
227 
231 
229 
245 
246 
228 
230 
230 
228 
237 
241 
304 
100 
122 
118 
103 
110 
111 
106 

211 
93 
28 
311 
25 
271 
53 

232 
75 

200 

1 

277 
185 
204 
210 

Diffraction 

Diffusion  of  Liquids  
"           Gases  
Distillation  
Divisibility  
Ductility  

Gassiot's  Cascade  
Geissler's  Tubes  
Gold  Leaf  

Alcoholmeter  

Governor,  The  
Gravitation  
Gravity  
"        Centre  of.  
"        Specific  
Gulf  Stream  

Halos  

Harmonics  
Heat  
"     affected  by  Rare- 
faction   
'      Absorption  of  
'     Conduction  of.  .  .  . 
"     Convection  of.  .  .  . 
'     Expansion  by.  .  .  . 
'     Latent 

Ear  The  

Arm,  The  
Artesian  Wells  
Atmosphere  
Atomic  Theory  
Attraction  
of  Adhesion. 
Cohesion  . 
"          Capillary.... 
"           Gravitation  . 
Aurora  

Barker's  Mill       

Ear-trumpet  

Elasticity 

Electric  Battery  
Light  
"       Telegraph  
"       Whirl  
Electrical-machine  
Electricity  
Frictional... 
"          Galvanic  
"          Magnetic.  .. 
Electrodes  
Electro-gilding  
"      -magnetism  
'      -magnets  
"      -negative      a  n  d 
positive     sub- 
stances   
"      -plating  
Electrolysis  
Electrolyte  
Electroscope  
Equilibrium 

Barometer  
Battery,  Bunsen's  
Grove's  
Sulphate  of 
Copper  
"        Thermo-e  1  e  e  - 
trie  
Bell  
Boiliia-  238, 
Britt  eiiesi  

Came:  a  
Capilli.rity  
Capstan  
Cartesian  D.ver  
Centre  ot'Ciravity  
Oscillation... 
Chemical  Affinity  
Chromatic  Aberration.  . 
Clock  61 
Clouds 

Luminous  
'     Mechanical  Equiv. 
•     Quality  of.  
'      Radiation  of  
Reflection  of  
Refraction  of  
'       Solar  
'       Specific  
'       Theory  of.  
•     Vaporization  
Heating  by  Steam  
Helix 

Evaporation  
Expansion  ...   
Eye    The 

Falling  Bodies  
Far-sightedness  
Filtering  
Fire-engine  
Flames,  Sensitive  
"         Singing  

Horse-power,  A  

Hydrometer  

Hydrostatics  
Hydrostatic  Bellows  
"           Paradox... 
"           Press  

Iceland  Spar  
Inclined  Plane  
ndestructibility  
nduction  264 
nertia  
nsulators  
sochrontus  

Joule's  Law  

Cohesion 

Coils   Induction 

Floating  Bodies  
Focus  

Color 

'     -blindness  
'     Prismatic  
"     Complementary.. 
Compass  

223 

204 
206 
2GG 

G2 
26 
271 
109 
31G 
315 
291 
124 
263 

266 
32 
250 

Force  
Pump  
"      Centrifugal  
'      Centripetal  
"      Composition  of.  .  . 
"      Molecula*" 

Compensation      Pendu- 

"      Resolution  of  .  .    , 
Fountains:  
Freezing  Mixture  
of  Water  
Friction 

Cords 

Kite  

Correlation  of  Forces.  . 
Crystals      38, 

Current,  Voltaic  
"        of  rivers  
Curves,  Magnetic  

Declination 

Galvanometer  

Land  and  Sea  Breeze.  .  . 

"      Adhesion  of  
"     Buoyancy  of.  — 
'•      Compressibility..! 

Light  

Composition  of.  .. 
"      Diffraction  of.  .  .  . 

Dew... 

340 

Light,  Interference  of.  .     207 
Laws  of  188 
Polarized  210 

INDEX. 

Northern  Lights  282 

Oceanic  Currents  254 
Octave  166 
Opera-glass                  .  .  .     218 

Sound,  Refraction  of.  .  .     155 
"        Superposition  of    168 
"        Velocity  of  155 
Sounding-boards  175 
Sound  Waves  152 
Speaking  Tubes  158 
Trumpet  158 
Specific  Gravity  115 
Flask..     118 
Spectrum,  Solar  204 
Spherical  Aberration.  ..     214 
Spheroidal  State  242 
Steam  241 
"     -engine  246 
Steelyard  88 
Stereoscope  219 
Stringed  Instruments  .  .     17U 

Tacking  74 
Telegraph  306 
Telescope  216 
Tempering  39 
Tenacity  2;» 
Thermo-electricity  3*3 
Thermometers  230 
Thunder  2.4 

'         Reflection  of  190 
Refraction  of...     197 
Theory  of  189 
Total  Reflection    197 
Velocity  of.  ....     189 
Waves  of  208 
Lightning  281 
Liquids,  Buoyancy  of.  .  .     116 
"        Cohesion  of.  ...       36 
"        Compressibility 
of.  26,  29 

Optics  185 
Optical  Instruments....     215 
Organ  Pipes  177 
Oscillation,  Centre  of.  .  .      60 
Osmose  of  Gases  46 
Liquids.....'       45 
Overtones  175 

Page's    Rotating     Ma- 
chine              .        ...     305 

"        Diffusion  of.  ...       44 
Elasticity  of.  ..       29 
"        Osmose  of  45 
"        Pressure  of  114 
"        Specific   Gravi- 
ty of  118 
"        tend  to  spheres      37 
Liquefaction  236 

Machinery  85 

Pendulum  58 
Perpetual  Motion  79 
Pisa,  Tower  of.  56 
Pitch  '  163 
Platinum  Wire  28 
Pneumatics  132 
Pneumatic  Inkstand  ...     147 
Polarization  of  Light.  .  .     210 
Heat...     228 
Electric- 
ity...    264 

Magdeburg  Hemisphere    136 
Magic  Lantern  219 

Magnetism  261 
Magneto-electricity  261 
Magnets  261 
Magnitude  25 
Malleability  28 
Mariotte's  Law  141 
Measures,  Standards  of.       15 
Mechanical  Powers.  ...      83 
Mechanics,  Principle  of.      85 
Microscopes  215 
Mirage  202 
Mirrors                                 191 

Pressure  of  Air  132 
"            Gases  132 
Liquids  103 
Prince  Rupert  Drop.  ...      40 
Prisms  199 
Pulley  97 

Torsion  Balance  31 
Tourmaline                          211 

Turbine  Wheel  126 

Velocity.  .  .  .                          67 
Vibrations  ot  Air  152 
Cords  169 
"              Ether....     189 
"              Pendulum      58 
Solids  ....     228 
"          Sympathetic    178 
Visual  Angle  188 
Voltaic  Arch  296 
Battery  292 
Electricity  287 
Pair,  The  289 

Water  255 

Pumps  142 

"       Air                             132 

Rain  252 
Rainbow                              212 

Reaction                                78 

Molecules  21 
Molecular  Forces  35 
Momentum  68 
Motion                                    65 

Reflected  Motion  79 
Relay  309 
Resonance  161 
Rest  67 
Ruhmkorif's  Coil  312 
Rupert  Drop  40 

St.  Elmo's  Fire  284 
Screw...  95 
Sensitive  b'lame  182 
Ship    Sailin"  of                    74 

'       Compound  71 
Circular..  ..  75 
1       in  a  Curve  79 
'       Laws  of  70 
'       Perpetual  79 
Reflection  of.  ...       79 
'       Resistance  to...       67 
Music  163 
Musical  Scale  176 

"     -barometer  142 
"     -level                          115 

"     -wheels  125 
Waves  128 
Wave  Motion  129 

Wedge                            .        90 

Singing  Flames  182 
Siphon  145 
Siren                                       164 

Near-sightedness  222 
Needle,  Astatic  303 
Magnetic  263 
Dipping  267 
Newton's  Rings  207 
Nodal  Lines  173 
Nodes  171 
Noise  163 

Snow                             .   .     253 

Welding       36 

Solution  44 
Sonometer  170 
Sound  151 
Intensity  of  157 
"       in  a  Vacuum.  .  .  .     155 
"      Interference  of  .  .     169 
"      Reflection  of.....     160 

Weight  49 
Wells                    112 

Wheel  and  Axle  90 
Wheel  work  92 
Wilde's  Machine  311 
Winds                  253 

Wind  Instruments  174' 

